Re: GTA IV ridiculously low FPS
Have you tried disabling video capture?
Oh hang on, are you wearing cotton pants (I'm English so you know what pants I mean)? I hear that can cause it too. It's just as likely as it being your GPU manufacturer.
Edit: I just noticed you stated it was working fine on your 4870, in which case you probably had video capture off already. In that case I would go with the driver reinstall.
System: Core i5 2500k | 8Gb G.Skill Sniper LV | ASRock z68 Extreme 4 | Sapphire Radeon HD5870 1GB | Creative X-Fi | Thermaltake Toughpower 700W | Windows 7 x64