The core of Playing a game today Most porbably would lie in the most obvious graphic card purachse,which many people seem to have a restricted Budget say about 3k to 4k which wont fecth them much more than the low-end budget card like Fx5200.Hence compromising on gameplay.So If you consider Gaming as your Major purpose (Both casual and Hardcore i.e.) consider having yourself with enough money left to buy a good Graphic card.Smooth Gameplay would depend on what settings you actually like playing the game on....If higher resolutions with most eye candy on then dont consider anything below a 6600Gt Or Radeon 9800Pro.
Hence the system can be squared off to a AMD Athlon 64 bit processor,a Motherboard Like Asus K8N,Minimum Of 512MB DDR Ram,80GB Hard Disk or if more needed than a Segate 160Gb One,Cabinet of course must have 400Watts of power,Keyboard and Mouse as per you pref,Monitor 17" Minimum CRT or 15" LCD and at last the graphic card must be "Atleast" a Ati Radeon 9600Xt Above type and Geforce 6600Gt would probably make The best VFM Buy.