The difference is nothing to do with 2.1 / 2.2

One card is 3.3v the other is 5v

So Can I use a 2.2 card with a motherboard that has 2.1 slot?

What does the motherboard manual say regarding pci 2.1/2.2/2.3?

See http://www.tomshardware.co.uk/forum/214004-10-interface

Thanks for your help mike10, tans thanks for the link.

This is what the pc spec's say

PCI Slot
- 1 x 32Bit PCI 2.1 Compliant card up to 24cm
- PCI 2.1 Compliant
- Supports 3.3V and 5V PCI cards

So if 5v=2.2, then 2.2 should work>?

5v doesn't equal 2.2

From the link:
"PCI 2.2 will do 5v or 3.3v, whereas 2.1 will only do 5v... per spec requirements
 
I know many will read this and say "AH! But PCI 2.1 WILL run 3.3v!!!" This is true to a point- the enabling of the 3.3v rail was encouraged in the PCI 2.1 spec. It wasnt until 2.2 that it was required. So, many MB manufacturers opted not to."

So basically if the motherboard states 3.3v or 5v then it should be fine.