Well the OP already has 6GB DDR3 RAM,and as for VGA both HD 5770 and GTX 260 are faster than a GTS 250,but like i said for 1440x900,GTS 250 can handle all games fine.
If the resolution was something like 1680x1050,then i would agree with you,because in those resolutions,moving from a GTS 250 to a higher card like 5770/GTX 260 would be worthy
When I played GTA 4, it was with a Q6600 @ 3.2Ghz and an 8800GTS G92 (arguably same thing as the GTS 250). I also played @ 1440x900 at the time. It was perfectly fine.
The GTS 250 is suitable based on his resolution.
I do agree though that his video card is "underpowered based on his whole system specs", but again at such a low resolution it won't matter.