^^^
Sorry, but the "upgrade" you suggest is not justifiable because the overall performance increase will be very minor compared to the amount of money spent on older tech (meaning the CPU).
To the OP,
Your choices are:
1. A new PC. This is the most expensive route, but with the right parts you will be able to play GTA IV decently. Roughly $600+ for the build without going into specifics.
2. As I stated before, buy the PS3 or Xbox 360. At least with this option you will not need to worry about upgrades for console games.
3. Live without GTA IV.