How do those x2 GPUs work on a non-sli/non-xfire mobo?

I have been wondering for awhile if you can use one of those x2 GPUs on a non-xfire/non-sli mobo and still have a performance benefit for gaming. This has been a question I've had in my head for awhile and I finally wanted to ask because I've yet to stumble on a concise answer. I know that those x2 cards have two GPUs on one PCB but I was wondering if they work correctly on a board with only one 16x pcie 2.0 slot that doesn't have sli/xfire support, or do they just show up as a single under-clocked(I know the gtx 295 is similar to two 275s on one board, and the new HD 5970s are like two under-clocked 5870s) GPU?

I have a mobo that only has one 16x pci express 2.0 slot and currently have a XFX gtx 260. I was contemplating getting a 5870 but those 5970s look nice too. My question is do the 5970s do their xfire-ing on themselves or would I only see the benefit of having a single underclocked 5870 by using this on a board without sli/xfire capabilities?

Here's my system specs in case this helps get a correct answer:

Biostar TF8200(socket AM2+) I believe that this uses the nforce 680i northbridge if I'm not mistaken.

AMD Phenom II x4 940

8gb(4x2gb) gskill ddr2 1066

XFX geforce 260 core 216

Corsair 750w PSU


Thanks in advance everyone.