I'd go with the 5750 over the GTS250, just for the sake of avoiding older tech. better heat and power consumption being a plus.
I play SC2 on my HTPC/light gaming machine, at 1080p on ultra settings and 4xaa 16xAF. the framerates are plenty playable, and that's with a 5770 and athlon II 635.
so at the lower resolution, im sure a 5750 would suffice, and even a 5670 would probably handle it. though i'd suggest the 5750 anyways, as you are better having the extra power.