You'll need to find a proper PSU for it, and in a dell SFF that might be tricky. To be honest, trying to find a PSU in a TFX or other SFF format is not an easy thing to do. Now, that being said, a GTX 650 has about half the firestrike score of a GTX 750ti.
AND a GTX 750ti -usually- doesn't require a power connector, it draws under 75w or so. That means it can usually be run off a 250w PSU without issue. And it's faster than the GTX 650 by quite a lot. So I'd ditch the 650 and go to the 750.