I actually used a Lenovo like that in a build. It has a 250w PSU, you should just scrape by on a 7750 with it. The case is plenty big for a card that small, I had a 7850 in it with bags of room. If you choose to upgrade your GPU in the future you'll need to either get a new case... or saw the lower hard drive cage out
I would be very wary about installing a Radeon HD 7750 in a PC with only a 250w PSU regardless of the manufacturer. Even Seasonic PSUs.
If in fact it is a 250w PSU, then the most powerful GPU I can comfortably recommend is the Radeon HD 6570.