www.newegg.com is by far my favorite computer parts website out there. excellent prices, excellent service, excellent products.
first and foremost, what a good videocard is is definitely dependant on what your intended use is. if you just want to 4 table poker and watch some dvds here and there, you don't need much at all. if you want to run half-life 2 with 8x FSAA and 16x aniso, then you're going to need the biggest baddest card out there. The same is going to go for the processor, you won't need an athlon 64 3700+ for 4 tabling poker. a much more modestly priced processor will do you fine.
Moreover, if you're willing to build the computer yourself, you can save a lot of money. However, you do tend to sacrifice tech support and extended warranties if you do this. If you're looking for prebuilt, i can't help you too much, as I don't have any experience buying prebuilt computers (except my laptop). However, I hear dell has good tech support.