is your card like this?http://www.amazon.co.uk/Club3D-GTX550-3072MB-PCI-E-HDMI/dp/B005GLXX5C/ref=sr_1_fkmr2_1?ie=UTF8&qid=1331381840&sr=8-1-fkmr2
difference: not much.
vram intensive apps will benefit ever so slightly from the extra gbs of vram.
might come in handy in sli if microstuttering is not a major issue.
decent upgrade from a gt 430.
low end physx, cuda, 3d vision card.
... can't come up with anything else atm.
gpu will likely choke long before being able to properly use 3 gb of vram. current more powerful gpu like 7970 properly uses 3 gb vram.
extra vram increases power consumption.
3 gb cards are usually costlier than 1 gb ones.
lower price, performance, efficiency ratios.
possibly get outperformed by a radeon hd 6790 or a 6850.