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 tho
There's night and day performance difference between these 2 even in 1,1.
But this is strictly app dependent. I've used number of different GPUs with my 1,1 and it's always like that. CPU limited ones will never show an improvement.
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.