You'll be fine. I've got a 6950 running in a PCI-e 1.0 board. That PSU is a bit wimpy I think, but it's a good brand and will probably be fine. You're just going to have to be careful about adding new hardware, since you'll be close to the limit of what that PSU can do.
To confirm, there is no drop in frame rates due to my board being a pci-e 1.0? The cpu wont be affecting it either? Or is it small differences of 1fps - 2 fps?