Well, generally, for a cheap computer, you'll get more for your money by buying a cheap, name-brand one.
While, for a high-performance computer, you'll save money by building it yourself.
Since yours is mid-range (I think; haven't checked conversion rates), you should probably look at both options.
How much video editing do you plan to do? That can be very resource-intensive sometimes.
When I buy a pre-build computer I'm given the impression that I'm buying the brand name opposed to the actual specs.
The exchange rate from AUS to US is .983 but the bastards here in australia love to rip us off. So it's relatively mid-range.
I guess whenever I get the time to video edit or when I have actual content that i'd like to compile together... Which is rarely but I do like the option of having a reliable system that can process and render HD videos.
What do you already have on hand? Can you reuse a case, power supply, keyboard, mouse, DVD, OS etc?
Is this to replace a system, add to a system or is it for a new location? How is this machine going to fit into your stable of pc's? What OS are you going to run it?
For your budget I would forget about the i series cpu's. You can find better performance per cost ratio processors that use less expensive mobo's.
2MB ram should be plenty to start with. You can add more later if you really think you need it. 4 is the max windows 7 recognizes anyway.
If you have the luxury of taking your time putting it together, then you don't have to buy everything at once. Get on Newegg's email list and every computer store near you. Pick up components that you want as they come on sale.
I tend to buy the cpu and mobo at the same time to make sure they are compatible. I buy the ram after buying the mobo again to ensure compatibility.
I bet you could find a kick ass used system for that kind of money. I routinely see them on my local Craigslist.
For a high-end computer, you are.
But on the lower end of the scale, it's actually cheaper, because the big companies buy in bulk. -- Their surcharge for the name brand is less than the amount you save from that bulk discount.
For mid-range though, it will probably be about the same either way, but it would be good to check.
Since you don't edit video very often, you probably won't mind longer waits while it processes the video, you you'd probably be fine with a cheaper, slower, computer.
(Depending on what kind of games you play.)
Personally, when I buy a new computer, I always get the one that was the best available -- 1 year ago. It will be built with quality since it was high-end, but it will still be cheap, because computers become obsolete so fast... And I don't mind living 1 year in the past technologically.
It seems a little redundant to have a separate desktop at all...
Unless your laptop is insufferably slow, why not just use it as your desktop as well?
(Posting from a laptop that hasn't been moved for months.)
For specific recommendations I like to read Maximumpc magazine. I value their opinions. They have a pretty good website too. Each month they update their recommendations on high, medium and low end components.
I would pick up a Solid State Drive to keep the OS and main programs on. Use the big TB drive for video and mp3 files.
Use amd processor and Hitachi hard disk its good to use and also cheap.
Intel's new Sandy Bridge line of chips just hit the market. These chips are definitely worth a look - the GPU is built onboard and by all indication it can handle light gaming. Prices are reasonable too - you could definitely work up a system and stay w/in your budget.
why not try AMD processor, Phenom II X4/X6 having a price range of intel i3/i5 thought they have the same clock as i5/i7 series. price wise it's better, the only performance that would be noticeable is when you're rendering more than 1080p movies with extra high bitrate, and when you're benchmarking. For casual gaming and typical stuff that you'll mention, they're on par.
i5/i7 intels ranged from 170-340AUD
AMD Phenom II X4/X6 ranged from 100-270AUD
for ram, I prefer OCZ / crosair, while a bit more expensive they really more reliable (performance, after sale service, etcs) price range from 90-130AUD I think for a 2x2GB package.
Mobo, get anything that's on the middle range of price that available in your area shop. ASUS, Gigabyte, MSI are the usual brands, I myself prefer BIOSTAR. Seriously, choosing brands for motherboard slowly turns to be like fanboys debating between PS3 and 360. price range 100-150AUD is good enough, just take anything that you thinks pretty XD
Case, once again, take anything that you thinks pretty and match your budget, sometimes high-end case would just a burden since your power supply got not enough wire to supply every add-on your case had.
HDD, why not take 1.5/2TB, the price difference between 1TB and 1.5TB is not that great, just 15-20AUD difference. you could get 1TB for around 60AUD, and 1.5 for around 85AUD Western Digital green series btw.
Graphic card might be the most expensive add-on you'll put, price ranged from 170-whooping 600AUD, just take anything you could afford from your remaining budget that you could spare. IMO ATI good for processing movies (decoding encoding) and overall performance while NVIDIA offers better support for gaming, that's from my subjective perspective and experience.
The really vital part you might consider is the Power Supply, since many case didn't come with premium Power supply (or didn't come with one at all), you might wants to switch it with some brand name like thermaltake, cooler master or crosair. Power supply usually a forgotten part when you're building a new PC, it could means whether you're pc running smooth or not, in their best performance or not. Actually my friends that already works in this kind area since 1995 gives me some trivial saying, "if you wants to build new pc, build it from the power supply, get the best part you could buy, you could buy and upgrade the other part when they're broken or out to date, but a broken power supply could affect the life and death of your over all PC performance"
I really recommended Thermaltake Toughpower for performance purpose, but refers to your shop about after sale service, and you might consider another brands.
sorry if I'm not giving an exact devices recommendation, but you could use it for a little more reference, right?