Goto your local computer shop, not a big company, the little one on the corner tucked away somewhere
They can build it for around $35-40~ extra.
IF your budget allows, heres what i think would best suit you..
Case: Whatever takes your fancy
Power Supply: Coolmaster iGreen 500 or 600 watt
Processor: Intel Core 2 Duo E6600 OR AMD Athlon x2 5200+ (is a bit cheaper, better option if you wont o/c (overclock))
Graphics Card: 8800 GTX (dose not matter what brand, Sparkles tend to be cheaper)
Mother Board: A good $150~ the shop will help you on that
RAM: 2 GB,
dont get generic! Get a brand name one such as Kingston, A Data or Crosair
Hard Drive: Seagate 320GB SATAII (or more depends how much storage you need)
This system would run all the new games for a good 6 months + with very decent settings
