I don't know if I would go with a 7750 with such a small PSU. I think you probably have the same PC as my brother and I was told that a 50w GPU is about the max that PSU can do, and I wouldn't even go that high. I actually threw a 5450 in his so he could play WoW with better fps (yes the 5450 is more powerful than the onboard), but you could probably go a little higher than that. I'm thinking a 6450 would be about as high as you should go, they pull about 35w I believe.
According to specs, the HD 7750 will only require 55W at max load. That is less far than the 75W required to be available on the PCIe slot alone and only a draw of 4.6A on the +12V rail. http://www.hwcompare.com/11727/radeon-hd-7750/