Yes it would be fine, not many games use over 512mb, Crysis 2 official requirements 'A GeForce 8800 GT or Radeon HD 3850 with 512MB of video memory'. Your better off going with a faster GPU than MORE memory. Just google GT240 vs GTS250 and then judge for yourself.
well at that resolution, yes the 512mb should do you fine. but saying that, at that resolution you wont see any difference between gts250 and gts240 as you will be bottlenecked by the cpu. Oh also, i did not see that before but you only have a 420w PSU of unknown brand(suspect its a cheap crappy brand) a gts250 requires more power than that psu can deliver, better stick to the more power efficient gt240 or your psu will blow up.