Get an ATI Radeon HD 4870 if you can afford it, but like Alvar said, if you're using a computer with a dual core processor, you're going to need to upgrade to a quad core to run games like GTAIV with reasonable framerates.
Some dual cores can Outperform quad cores in certain games, but with GTA IV it is not the case.
What you need is a new computer if you plan on playing modern games on your pc, or at least an upgrade (sorry i didnt read to see if you listed your specs)
A cheap 70-100 dollar graphics card isnt really going to do you justice unless you just want increased framerates in games in san andreas. The standard playable FPS for enthusiast gamers is 60 FPS.
Anything below 60 is considered laggy and unacceptable, but if you are content with that then you can start looking at cards under 150 dollars. Anything below that price is going to be struggling against modern games.
