Just to give a rough idea. All numbers look to be from Tom's Hardware, so they're canned benchmarks.
Looks like it depends on the titles you're considering, but over all the 9800 is faster. In newer shader heavy titles the 5750 may pull away with a win. I'd personally sell both and get something like a 5850 / used 5870.
9800GTX+.
Like silent-circuit said
if I were u I would sell both and get 5770 if you dont want to spend extra money
or used 5850 for additional $100(guessing)
Sell both and get a 1GB video card. A lot of the modern games need 1GB of video memory for you to crank up all the settings. Maybe a 5770. If those were free... then it would depend on what games you play. I'd give the 5750 the edge for having DX11 but the 9800GTX the edge for games that feature physx support.