I would recomend building your own. It is pretty easy to do and you won't be locked when it is time for an upgrade.
You will get far better quality and probably save some money in the process.
To run vista 64 you will need a 64 bit processor core a crap load of ram and a decent video card.
i don't think hard drive size is relevant, anything over 100gb should be fine?
For extra storage check out the Drobo from data robotics.
for specs, go to some overclocking forums and they should be able to help you out. and if you need it now, then core 2 duos are in for a big price drop by the end of the summer so just wait for it. and 4gb of ddr2/ddr3 if you can afford ddr3. and graphics, one of the pricier nvidia ones will do fine, juxst make sure its for gaming not graphics.
motherboard i have no clue.
but a better solution than to ask at this forum would be to find an overclocking forum, cause people there know alot more about this sorta stuff (as you can see).
hmm thanks for crushing my spirits mate i didnt need that ego anyways.
BTW what wud be a good dual boot program?
My suggestion for the new generation PC would be to make sure you aren't nearing your limitations. for example, with a 32-bit OS you are limited to 4GB of ram. But with a 64-bit OS the limitation is 8GB. 4GB is almost common today. So people are eventually going to have to move to 64-bit. Just a thought.