saying want low res and crysis to look decent is an oxymoron.
I take it your just upgrading onto the spec in your sig. is that ddr2 ram? whats your complete spec details? then maybe we can help you more.
But as it stands the guys are right.. you just throwing your bonus away on that card lol sorry. you graphics card aint so bad.. its 1gb ram and pentium D thats letting you down. i suggest overclocking for cpu to 3gb.. if a good cooler is needed get one good one can be found for £10-17 Artic freezer pro 7, get another gb of ram.. and see if you can mildly overclock your graphics card... Save the rest of the money and put it towards a new build. just popping a dx10 card in that setup aint gonna make it wonderful your gonna have bottlenecks. what HDD you got ide.. maybe get yourself a big sata one.
thats my suggestion, goodluck whatever you do m8