GTA IV likes plenty of memory, both system RAM and video RAM, which leaves the 512Mb HD4770 out of the picture, even if you could find a pair.
That motherboard will not support SLI, you're better off sticking with ATI.
Without a doubt, the 1Gb HD4870/HD4890 is your best bet, it's fast enough and leaves Crossfire an option later.
For Nvidia, I'd go for the GTX275, it's close to the GTX285/280 for a fair bit less and faster than a HD4870/4890, but then again, there is no upgrade option short of replacement and it's more expensive than a HD4870.
------------------------------Peace cannot be kept by force, it can only be achieved by understanding: Eienstien
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