URGENTLY in need of some HELP!

What computer companies (one's that are OEM/pre-built) love to do is overload on the CPU side and barely do anything with the GPU part, letting them use a super low-watt power supply at the same time.

A core i7 does not belong in the same computer as a radeon HD 75xx GPU. It needs at LEAST an HD 77xx GPU, but honestly, you'd do better to pay even more for a 78xx series (79xx series only if you have the money). A 78xx series still won't play FC3 on ultra (at least I don't think it will).

Usually sales people in places like Best Buy or Fry's don't actually know what they're talking about. I've asked them specific questions that I already knew the answers to, just to test them, and they get them wrong or don't give the better option 9/10 times because they simply don't know and aren't trained well.

Get the HD 7850 if you can manage to increase your budget a little bit. You won't regret it at all. Otherwise go for either the HD 7770 Ghz Edition or the GTX 650 Ti.

Edit: The 1st of the two pre-built computers you just listed is better than the second. I assume it comes with a 500W PSU like the 2nd one, but make sure you ask them at Best Buy. If it's 500W, then you can and should replace the GPU with something better, since you are shaving off ~$150 from the one you already bought. I would increase your budget a little and get the HD 7850 or 7870 if you can and you will not have to buy another PSU (but shouldn't overclock anything). The reason you shouldn't overclock anything is because the PSU's they use are pretty cheap and aren't built that well. For regular use, they should be okay though. Down the road you might want to watch for sales on PSU's and get something with an 80 PLUS Bronze rating from a brand like Antec, Rosewill, or Corsair. Other guys here on the forums will know more.

Edit2: Also to answer your 2nd post, you would still need to upgrade the GT640 as it is still a lower-end card.