I say stay away from that 9950. TBH, I think the 720 is the best among the lot, but you cant go wrong with the 635 either. The 555 is good and can be unlocked to a quadcore(so can a 720). Just make sure your motherboard has the ACC feature.
Please do note, the extra core that will be unlocked may or may not be stable. Hence I think it has to be between 720 and 635. You might also want to take a look at AMD Athlon II X3 440 which retails @ around 70$. If 100$ is too much for you, go for that. Else, 720 or 635 are too good @ 100$. GL. And, about your GPU, how much are getting it for? You can probably wait a month and buy a 5770 for 130$.