GTA 4 is badly coded... In some article, I remember, i read that there was a small difference in the performance of a 9800GT and a GTX 280.
Overclocking your processor might give you better performance in GTA4.
Crysis needs GPU muscle.If you think upgrading for a single game is worth it then go for GTX 470
Otherwise , I would say, GTX280 is still a very powerful card and you can use it a little longer. . .