For modern gaming, d most important specs r processor(CPU), graphics card (GPU),& RAM.
Make sure ur mobo can support ur:
i) processor,
ii) type of RAM (ddr2,ddr3,etc)
iii) graphics card (a 512mb ddr3 would work fine, dont get ddr2)
iv) power supply to support ur GPU
Since i'm using intel cpu & nvidia gpu, i'm not sure bout AMD CPU&GPU.
For acceptable gaming experience, higher RAM & processor speed means better performance. Also, for gaming, u should use Windows XP SP2. Don't use Vista, as it uses up too much memory (u need to upgrade memory just to get acceptable performance).
Windows XP can support max 4gb RAM.
Look for d GPU speed (eg: 650mhz, higher is better) & ddr3 (again, higher's better) when looking for GPU
I'm using Intel Core 2 Duo 2.8ghz (different frm Core Duo/Dual Core, as these 2 r d lesser CPU), 2gb RAM Corsair ddr2, 9500GT 512mb ddr3 GPU (entry level gaming GPU) & I'm playing BatmanArkhamAsylum, Far Cry at high settings & no lagging!
ps: My system is a moderately cheap built for normal gaming. & d 9500GT uses only d standard 500W power supply unit. Didnt go for Quad Core as it costs more..