Use fps configs for games etc. Also Force your dxlevel in game to a lower version such as directx 7 (some anti cheat's will kick). You will soon see a fps increase
If you don't try and if you don't expect to play GTAiV on that, try to stop as much windows services as you can, defrag the registry, stop any program that runs in background(antivirus too, but just when you are playing), even internet if you can :P and then you may play a ... "decent" game.
for a reasonable amount of money you can afford a 256 or 512 mb video game cards
but i would advice you to rather buy a game console , the xbox360 is not very expensive , and you won't ever have to change your graphic card to play with it's game