If at all possible don't buy a prearranged system. Pick out your own components. Read online reviews to help you decide on the components. I did this myself just recently. After I decided what it was that I wanted I emailed a few places that sold the components that I was looking for at reasonable prices with the list. I told them that I wanted the comp assembled, tested, and shipped and asked for a quote. Prices can, and probably will, vary by hundreds of dollars. I didn't play any games. I just took the best offer and went with it.
You have to be a bit flexible. One place might not have everything that you want in stock. I told them it was OK to substitute with comparable items if they did not stock a particular item on my list. Different manufacturer with the same specs is usually fine.
What you want depends on your main use for the computer. For gaming the video card is crucial. Don't scrimp there. You can get excellent cards for 150.00 or so. I'm not in the US so I'm not exactly sure about pricing there.
Intel is making the best cpu's at the moment. Because of this AMD has dropped the price of their processors pretty dramatically. For a straight gaming machine you can get an excellent AMD processor that's about 90% of the performance of an Intel for about 1/2 the price.
If you're going to over clock the system and/or are looking for state of the art performance, go with an Intel. You'll need a top of the line motherboard and after market cooling as well though for over clocking. This will increase the price by quite a bit.
Nvidea is the best choice for vid cards right now. The new 9600's are a great value.
If you're going to go with dual cards I personally like ATI crossfire (better motherboard selection), but I'm sure you can find just as many proponents of SLI. Dual cards though aren't the best cost for performance. 2x the money but not 2x the performance. The motherboard will be more expensive as well.
RAM is cheap. Buy a lot of it.
4gig at least, depending on your OS. Buy good RAM, but don't go overboard. Real expensive RAM isn't going to improve real world performance over good ram. For example, I bought G.SKILL 4GB kit (2GB x 2) DDR2-800 (PC-6400) CL4-4-4-12. It's about $100.00.