I don't think they're actual 5870 boards. The deal is that ATI haven't issued a reference design for the board as they usually do, so the card manufacturers can pretty much so what they want. The powercolor ones use the same board form as the 5870 because (I think) this allows them to use the same cooler. I don't know for sure if they use the same boards or just the form though.
The XFX ones look a lot shorter, it could be a custom board form or even the 5770 form.
Cheers for the advice on games by the way Spririts, I've just been monitoring mine, it went to 50C but was only using 21% fan speed, so I assume it has a fair bit more power to give.
EDIT: From watching the stats change, it looks like 21% is the lowest the fan goes at all, so mine was basically idleing under COD4. Tommorrow when my HDMI cable arrives I'll be able to get my second monitor working again and monitor the load and temps properly.