This card would run GTA 4 but I don't know if it will run with the psu that you have , can you upgrade the psu. It gets tough when people want to run these new games on video cards that have been around for a long time and are under $100. You end up playing at the low settings and resolution and in some cases not playing at all.
i know you can run a card under the recommended watts, but 280 seems to be pushing it. you should probably save up and buy a decent power supply first.
Forgot to say, my PC is a Dell Optiplex 745 DT, which only has one unique PSU, and it is low profile I suppose I can go redneck and run a PSU outside my case.
So you also need a low profile card for the thing also. Dont' know if those 77xx series come in that form.
Sorry to say... It may be time to save for a new inexpensive build. Don't go Redneck. Getting new parts one at a time can help you feel better about finishing. I was just there. It ended up being fun. Good luck.
Judging from your last post I get the impression that you have a slim line computer case and if that's the case then you can get power supplies for it. The one listed above is a 400w but there are others that are lower , 350w ,300w and so on.