Ok, forget what everyone says for right now except for Mr Samsa because he knows his stuff (and myself, of course). He's right about the 8800 gt. They just released a rather affordable 512mb version of that card that many are raving about. As far as ram goes, look for the fastest speed that will work for your motherboard. If you're going to build the computer yourself, aim for a motherboard that uses DDR2 800 ram. That is one of the "sweet spots" as far as "bang for buck" is concerned, and that will leave you speechless with the results you'll be getting. You should shoot for 2gb of ram, but hey, if you're going to use a 32-bit operating system, you can put 3gb in there pretty safely. Most computers that you buy at the store will have a 32-bit version of windows, and on those systems, 4gb of ram will not fully show up because it subtracts your video memory from the total system memory, so don't make a poor investment in that department. Also, if you're going to pick a computer from a store, you should make sure that it has 4 dimm slots (used for memory expansion) and at least one pci-e x16 or pci-e 2.0 (which runs at x32) for a graphics card.
Another essentially vital component of the computer that you have to worry about, if you're buying a store-bought computer, is the power supply. Most computers will come with a weak power supply that will supply only enough power for the base system. If upgrading the graphics card, you should really upgrade the power supply. If you buy the suggested 8800 gt (512mb version by EVGA is my recommendation), you should really look for a power supply with 500-600W that has dual +12 volt rails that each, combined, operate at a around 32A. The higher, the better.
I recommend the EVGA brand for graphics cards because they have a 90-day step-up program that will allow you to send your card back to the factory and allow for an upgrade. You may have to pay the difference in the prices. The program works like this. You send it in, have about a 2 week wait, and you have your new graphics card that you requested. If you pay $220 for your graphics card, you officially bought a $220 coupon from EVGA

. If the upgrade costs more, you'll be required to pay the difference, but I'm not sure if they will give a refund if the upgrade is a lower price... would be a good question to ask them though. Prices on graphics cards have dropped a bit recently and I expect it to continue.
Sorry for chatting your ears off though, hope I helped.
-DarkPacMan77-