They have different chipsets, the UD4P has the 790X while the UD5P has the 790FX. The differences between them are below(copied from my thead in my sig)I'd have to say this board is the best Gigabyte AM3 board out right now.
GIGABYTE GA-MA790FXT-UD5P
The only difference I see between this board and the UD4P is the UD5P
has more SATA ports.
790FX:
x16,x16 Crossfire | x16, x16, x8 Triple Crossfire | x8, x8, x8, x8 Quad Crossfire
65nm Process
790X:
x8,x8 Crossfire
65nm Process
Just to note, 790GX is x8, x8 as the 790GX only physically has 16 pci-e lanes.I suggest getting an AM3 chip, and an AM3 board. 790FX is for Crossfire with 16x lanes per card, and 790GX can have Crossfire with 16x/8x, and has onbbard video.