High at 1680x1050-1920x1200 resolutions with around 40fps ,um games like crysis 2 are say to run at 35fps average on 1080p on high settings on the nvdia website. The 6870 would get you up to 55-68fps on avg,that's how it seems to run on my rig with it all on ultra.
You can take a look at it GeForce GTX 550 Ti | Performance | GeForce
The only thing that would worry me though about the psu is that it has only 1 PCI-Express Connector
1 x 6-Pin and the 6870 would need two of them,I think you could get it to work with a converter but you could get it to work if you got a Molex to 12V 6Pin Adapter.