I'd say get another 4870X2 if you can stand the noise, heat etc and your PSU can handle em.. This heats my room up really quick, i'll most probably clutch at my heart whilst gaming and die this summer with the heat lol. I've not played GTA4. I'm curently playing COD5 left4dead and fallout3, i've completed Crysis + all of the expansions - once with 1 4870X2 and i played em all over again with the 2 in CFX and enjoyed it far more!.. Thinking about getting FEAR2.
Its worth me mentioning i ordered a GTX 295 (during the hype) then my m8 bought one and we found the hype to be total bull and far from the truth and in real world it wasn't anywhere near what the hype was making out. I cancelled my order for the GTX 295 and got another 4870X2. Best decision i made all year! like i said though.. Your PSU will need some beans and you'll need to be running a pretty decent OC on the CPU to get the most out of these beasts