GTA 4 has officially been released for the PC, but users are already having problems with their ATI graphics cards. With the unpatched retail version we have in our office has neither HD 4870 with 1024 MiByte nor a HD 4850 is running. GTA 4 is also denying to run with an HD 3870 or HD 3850. All four graphics cards were tested on two x48 systems with Windows Vista x64 and the Catalyst 8.11 WHQL. Right after the welcome screen GTA 4 crashes.
It is of course possible that GTA 4 (PC) will run under different conditions (operating system, graphics card, driver, motherboard). But there are reports of users in some forums and in the 3D Center forum that describe similar problems. It also seems like Nvidia cards are also negatively affected; they had to run the exe file several times before GTA 4 started.
Therefor we have to currently keep in touch with AMD and Rockstar and try to find a solution for the problem. Possibly a patch or update a driver can help.