The GTX 560 Ti is also worth going for. I'm getting the Core i3 2120, and GTX 560 Ti myself during this week. I'd say it would handle 1080p and 720p. I'll run benchmark tests once I have the GTX 560 Ti @ 1080p with GTX IV.
Its an old game, it doesn't take much gpu power to run it at high detail 1080p. The only problem is its poorly coded and requires lots of ram/vram and cpu power. I was playing it fine on an old 8800gts and core 2 duo 3ghz with 4gb ram, so im sure a newer core i3 and 1gb mid range or better vid card will run it just fine. You will find that it has the ability to use more than 1gb vram if you increase details too much, but it looks fine with slightly reduced draw distance.
GTA 4 plays fine on my Radeon HD 5850 at 1920 x 1200 resolution. The current HD 6850 is a little slower than that and the HD 6870 is a little faster than the HD 5850. Either one should be fine. I think the HD 6850 can be bought for around $140 after rebate.
If you want to play GTA 4 at 720p then a Radeon HD 6770 should be more than enough and it might be as low as $90 after rebate.