From my experience, gta IV runs very very very very poorly on PC. Even with crossfire amd 6950s, I still don't get 60 frames per second constant... Portal 2 runs very well from what I've heard from friends. I have a friend who has a laptop with a 5650m and he said that he gets great framerates with everything turned up at 1600x900.
What are your system specs? I don't want to recommend something to you that your system couldn't handle... I saw your configuration info, but you don't list your power supply... What wattage is it? Also, do you overclock, etc. I just want to get an idea about how much power your system can handle.
Between the two that you listed, I would go with the 6750, but I honestly wouldn't go with either of them. Have you considered the 550ti? It's almost the same price as the 6750, but has better performance. This is a great time to buy computer parts. Parts are soooo much cheaper than they have been in the past.
You could also go the used video card route (if you can buy the card from somebody that is reliable)... I just upgraded my gtx 260s to 6950s so I'm going to sell my gtx 260s soon. If your interested just let me know. I was planning on selling them for between $80 and $100... They still work perfectly. I just upgraded so that I could have some more gaming horsepower. (I'm a bit of a graphics nut, as you can see from my specs in my sig...
) If you have the dough for it, the 6950 is a REALLY REALLY REALLY awesome card, by the way. (I'm gonna sell you my 6950s. I love these things. If you want to get 6950s, you'll have to head over to newegg. I'm not selling mine...
********(If offering to sell something is not allowed on the forums, PLEASE DON'T BAN ME! I don't really know if that's against the rules or not. If it is, just tell me and I will NEVER do it again. I'm just trying to give FC360 some options....