Upgrade that cpu ASAP. If your mobo supports it, get an Athlon II X3 or X4 for a cheap upgrade. If not, then perhaps look at Pentium Dual Core + H61 mobo + 4GB DDR3, which can be had for fairly cheap. If you have more $$, then stepping up to i3 or i5 and 8GB ram would be even better, but honestly with a 550ti I think an Athlon II or Pentium Dual-Core would be a decent match.
The Athlon X2 (pre II series) is just a terrible match for modern gaming.
All of the above assumes that you'll give up on GTA4. It's horribly coded on PC, and with a 550ti you'd want to combine that with at least an i3, 4GB DDR3, and details turned down quite a bit to make it playable.
The rest of the games you listed will play nicely at medium settings on an Athlon II with a 550ti and 4gb. (2gb will work, but I find many users have tons of apps/giant AV products/etc running in the background, in which 4gb helps greatly in not running out of ram).