My old (3.5 years) motherboard AopenAK77 pro does not state what type of pci slots it has nor can I find referance in the manual or website. I was after buying a usb2 pci card for it but noticed that there where 2 types available, pci2.1 and pci2.3. Are they all backwards compatible or is there a difference in voltage / pins? If they were backwards compatible would having an older pci negate the benefit of usb2?

They should be backwards compatable.

Here's a link, this is way too much to attempt on the forum.
http://www.microsoft.com/whdc/hwtest...ype=err&ID=826

PCI cards are generally backwards compatible. The USB2 card should run absolutely fine on the motherboard, I can't see there being any issues at all with that.