I've been looking into ordering risers from ebay for my rigs.
I use this motherboard:
http://www.asus.com/Motherboards/F1A55M_LX/and 2x MSI Radeon 7950.
The specifications of the motherboard:
Expansion Slots
1 x PCIe 2.0 x16 (blue)
1 x PCIe 2.0 x16 (x4 mode, black)
1 x PCIe 2.0 x1
1 x PCI
I'm not sure what kind of risers are needed and what the difference between them is?
How many GPU's can I use with risers on this motherboard?
Looks like it can hold up to 3 assuming the BIOS lets you use that many. Unless you're getting it for a steal, there are probably better boards out there. For risers, you'd need 2 of the x16s and 1 of the x1s. You would probably want to run at least one powered riser for insurance against overpowering the motherboard (preferably an x16 that is powered).