I'm running an optiplex 745 desktop with a half height/low profile sapphire radeon 4650 ddr 512 mb, which I think is comparable power usage to the 6570. I've been running that fine for over 2 years now. I've even upgraded the cpu recently from the c2d e6400 to the c2q q6600, but that was done on a lark after I already bought a new sandy bridge based computer last summer. Still no issues with power supply on the optiplex, but like I said, it's not my main machine right now as it is just attached to my living room TV and I am not stressing it fully.
I have the pcie riser, but have not used it. I tried to put my 4650 (I still have the full brackets for my card) and a pci wireless card in the riser and it wouldn't turn on. I also tried it with the 4650 and the riser and it did work, so it seems like there might be a power limitation coming close with the 4650 and the q6600 and that extra wireless card might just be too much; alternatively there might a limitation through the riser. I didn't bother to investigate further as I just stuck in a usb wireless N stick and gave up on using my old pci wireless G adapter.
I do have a full height 7750 in my main desktop machine, and might slide it down to the optiplex 745 if I get a better card for the main desktop machine. Probably won't bother, though.