how can i make my pc a gaming machine. i dont know much about pc's so would appreciate help on what to buy.

according to my brother i will need a ne wgraphics card, as mine is really old. i bought the pc real cheap from my bro's mate and has stood me well for office stuff i have used it for so far but would like to upgrade it for gaming. will give you a run down on the spec and if any of you guys can recommend what to buy would be much appreciated. i'm not too sure on graphics cards etc and dont know how they work (slots, agp 2 4 6 8 etc?????)

Spec:

AMD athlon 1.4ghz processor
512 mg Ram (2 used, 2 free slots)
40 gig HD
Rage fury graphics card (has ATI rage 128 on it?) AGP slot is brown?
on board sound card

i'm running xp.

any advice guys. please dont say buy a new pc as i dont have the cash and only want to upgrade to play games, doent have to be an uber-powerful pc