well actually its possible. i used to have a 1.1ghz athlon, 128mb ram, tnt2 pro and then i upgraded to 512mb ram and a 9600xt and i can now play HL2, Far Cry, UT04 fine. you should definetely try to get another 256mb ram if not more and a 9600 pro or something. then if you OC you CPU to say 1.2ghz, you should be able to play modern games fine.
EDIT: but if you upgrade your CPU too (if your mobo can handle it) then you'll be even better off. but it would be smarter to get a whole new system then upgrade every component with still out dated parts... and you would need a PSU to power all this stuff, so if you can afford, just buy a new system