The UD4 has two PCI 2.0 x16 lanes, as does one of the UD3s, but the bandwidth for the UD4 is at x16,x8 (first lane, second lane), whilst the UD3 only has x16,x4 (first lane, second lane), so the UD4 would be better in crossfire.
If you don't intend to go crossfire later on, then the UD3s will be fine.
Scroll down a bit on these pages to see the PCI specification:
-GA-P55-UD4
-GA-P55-UD3
-GA-P55-US3L
If you don't intend to go crossfire later on, then the UD3s will be fine.
Scroll down a bit on these pages to see the PCI specification:
-GA-P55-UD4
-GA-P55-UD3
-GA-P55-US3L