Stay away from Phenom 1, they are bad. Your best bet out of what you listed is the 720 BE. If you want to try your chance at unlocking cores then the 555 BE is a solid choice. Even the 635 is a good budget choice.
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$. GL. And, about your GPU, how much are getting it for? You can probably wait a month and buy a 5770 for 130$.