Ok my board run in 16x8x8 or 16x16x1 mode. I have the 5870 in first 2 slots. Would I benefit from getting a cheap nvidia card for physyx in the 3rd slot?
It's probably not worth it. Very few games use physX. But if you were to add a dedicated physX card, 16x8x8 would be best. A 5870 would not be hampered by x8 bandwidth.