good processor, core i7 920 2.66Ghz, 2.93 and also 3.20GHZ
---intel is the best, yet AMD is also very good and cheaper, AMD PHENOM II is also quite good)
good graphics card, eg. ATI RADEON 4870 or above (4850 is ok though), perhaps put two of them in crossfire.
(Nvidia is also good, but are gooddam expensive for what you get, --- ATI is superb and fantastic)
Lots of ram = 4GB + for vista, 2GB + for XP
(more Mhz the better (like 1066 or 1600Mhz, any brand really)
make sure that your HDD is less than 50% full, because it makes your comp slow (7200rpm at least for uber gaming
make sure that you have an adequite power supply unit ( 1200w better)
go here for alienware, i think theyre silly design but quite good and very good value - aus web ::::: http://www.alienware.com.au/dnn2/
dell also make awsome gaming computers, make sure that theyre XPS though:::: http://www1.ap.dell.com/content/products/category.aspx/xpsdt?c=au&cs=audhs1&l=en&s=dhs