here's a recent bench of gta 4 with recent gfx cards:http://www.tomshardware.com/reviews/radeon-hd-7870-review-benchmark,3148-11.html
you didn't specify budget, location/preferred website (of purchase), psu specs etc.
you a 18.5" display, a gtx 560 or radeon hd 6850 should be fine. 6770 cards can run gta well, they tend to cost less as well.
edit: also list available choices.