yep, aces is right, increase your ram first & then get a used 9800 pro for arnd 5k or add some more & get used 6600gt, look in our market section for great deals, or start a new thread
remember your system is a bottleneck for newer games even if you get a decent graphics card.
If you want to play new games with all the bells & whistles turned on, then do as chaos said, time to upgrade your system