AMD e4850
X2@2.5ghz (haven't needed to OC it)
4gb CORSAIR DDR2-800mhz dual channel memory
SAPPHIRE HD4830 @ 600/900, or 700/900 if needed
Compucase HEC Continuous 600w PSU, 30/22amp 12v rails
ASRock A780FullHD microatx MoBo
500gb Seagate Barracuda
Excluding the HD, which I got for nearly free on a giftcard from my school, the whole lot cost me about ~$300 USD. Even though its running slightly below spec compared to reviews (probably due to the processor not being OCed), it still tends to max everything out easily at 1440x900. And I mean REAL maxing ;P And the board is wonderful... when I get more money, I can stick a Phenom II in it
I typically get 40-80FPS in intense sourcemods on max settings with 4xAA, and 120-160 with spikes to >200 on Gmod and HL2 singleplayer. About 30 FPS on E:TW with everything on high/ultra, with AA on. Lots of FPS on Company of Heroes with everything on ultra and AA turned up. CPU bound on Arma2, but I haven't bothered going higher than medium with high shadows and post processing, as the game is gorgeous, if impossible to play due to bluring at distances.
To the OP, please god not an Alienware. Unless you can't buy parts cheaply in Europe... otherwise, you're getting ripped off. A self-assembled computer is far cheaper for the same level of quality.