Yea the difference is huge, when I had my 1,1 It had a 7300 gt and was running a 30" and a second 1080P display and then when I got the 5,1 with the 5770 the difference was amazing, can't wait till I out a GTX 670 thoNo its replacing a NVIDIA 7300 GT 256 mb. in a 1,1. I just got a EVGA GeForce GTX 680 for my 5,1
There's night and day performance difference between these 2 even in 1,1.It will work, but I've previously seen that on a 1,1 the performance improvement over a 5770 is very small, unless that has changed.
If you see data where the 5770 is identical to the 5870, then the issue is that it's CPU limited. It's not a question of PCIe bandwidth, it's simply that the app/driver can't feed enough work for the GPU for it to make a difference.