I say your best upgrade is a GTX 260 core 216 896mb
You can find it for about 150 bones with rebate included etc.
start @
www.frys.com and If you have a local Fry's go there! gl
I personally wouldn't pay more then 150 to 180 bones after rebate.
FERMI is around the corner and this card will drop to 100 bones probably. gl
If you can wait one or two more months then you can get a 260 for even less.
Play the waiting game. Your current card on your rig is good enough for every game but 2 games.. Crysis and Far Cry . If you want to play those two you have to reduce textures and reduce resolution to 1280x1024 ...
Every other game your video card will pown. UT3 engine games,, Call of Duty engines,, MW2 will be ultra fast,,, its just this Crysis engine which no video card can handle still.
TIP: Use antiscropic filtering to 16x as its free. Turn off AA .. set to high quality and single gpu performance , in the nvidia panel.
In every game but Crysis engines you can go 1920x1200 all settings on and details to the max. If your obsessed with Crysis and Far Cry and future crysis games then a 260 wont be enough either. You will need a Fermi for Crysis and Far Cry to max out their settings and max out res... thanks.