Ok got a GTX 580 and chucked it in last night.
I ran some benchmarks before and after, the results are a bit confusing, I only changed the vid card between runs(drivers are latest Nvidia ones)
3DMark06 - Before around 12000 - After 17000
Vantage - Before around 7000 - After 21000
Dirt 3 - Before avg 33 fps - After avg 100 fps
F1 2010 - Before avg 34 fps - After avg 48 fps
So two are gving me around x3 increase and 2 are giving me around x 1.5 performance increase.
Confused as to the differnece in performance increases.