As far as overheating as long as the case itself has decent ventilation and nothing is cramping the gtx275 it should be fine. Frankly, they wouldn't sell it if it didn't work right.
With regards to the memory differences, i think as the resolution goes up the memory bandwidth will become more and more noticeable, but even when looking at 1680x1050 which is close to HD, it's rather clear the 275 is quicker, regardless of the memory difference.
I do wonder though if running multiple monitors would make a difference? Hopefully someone else will chime in here in a bit 8)