I will post my answer from your other thread, you have already asked this question many time before.....
As I have suggested many times in your previous threads, you will need to upgrade your PSU to even think about using a GTX 550Ti and even forget about the GTX 550Ti as the HD 6850 crushes it for around the same price too. GTA 4 is very GPU and CPU intensive. Unless you upgrade your PSU, you will only be limited to the HD7750 as the most powerful card your computer can handle. There is no other way around this unless you upgrade your PSU or want your computer to go up in smoke and potentially your house go up in flames too. 😉