I just bought 2x1Gb PC2-4200 Kingston sticks to replace 1x512mb stick that was in one of my desktops. When I put both new sticks in and boot, it freezes at the initial compaq screen. But jus one of the new sticks works fine. I don't know what could be wrong so any advice is appreciated. Thanks!
Check your motherboard's documentation, you may be installing too much for one slot.
It could be that you've gotten bad RAM... Getting bad parts straight from the factory happens frequently in my experience, but you can almost always exchange the defective ones for good ones.
Does just one stick work no matter which stick that is, or is one stick making it freeze, while the other works correctly?
I think motherboard memory limits are on a per-slot basis. (So a 2GB max motherboard could only handle 1GB per slot), so I don't think it would make sense for just one new stick working to be caused by memory limits. (Also, I think that if you put too much in, it should fail to recognize the extra, instead of freezing.)
If you have access to a POST probe, use it.