Intel Core 2 Quad 2.4Ghz O/C to 3.2Ghz
ATI HD 4870
Asus P5k Premium
In outdoor scenes i get between 30-40 fps @ 1650 x 1050 with everything upto 100 except for texture quality and view distance.
If you can overclock the cpu it will be gain you more fps as it uses more cores.
Output from GTA4's built in benchmark -
Average FPS: 41.24
Duration: 37.73 sec
CPU Usage: 66%
System memory usage: 58%
Video memory usage: 75%
Video Mode: 1680 x 1050 (60 Hz)
Texture Quality: Medium
Render Quality: High
View Distance: 33
Detail Distance: 100
Microsoft® Windows Vista" Ultimate
Service Pack 1
Video Adapter: ATI Radeon HD 4800 Series
Video Driver version: 126.96.36.1990
Audio Adapter: Speakers (Creative SB X-Fi)
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz
You should have no problems running the game on that config.
However after reading a lot of post about this game you might run in some troubles with starting the game. I personally didn't have any issues with the game, but some people say that are not able to get the game running or have really low FPS. But I think this is just a software or driver question. The machine you described should be OK.
No problem. im running Athlon 6000+ X2(no need for quad core), 3GB DDR800, Geforce 9800GTX+ Superclocked. Running 20-50fps depending on situation in game. At 1680x1050 and everything high except Texture Quality.
Don't expect to max this game out. I don't know what resolution you play at but if you game at 1920x1200 with a 9800GTX+ 512MB you will be limited to a Draw Distance of about "11" and Texture Quality of "Medium". If you really want to get increase the texture quality and increase the draw distacne get the 1GB version. Depending on your rez with that setup you will probably average in mid to high 30 fps.