the P45'll give u x8/x8 whn in crossfire and the x48'l give u x16/x16 if ur going to do crossfire then i suggest u get the asus, but if ur going for a 1 card option, the gigabyte looks better. btw get the 5770 its less power hungry and better at its job
wat board manufacturer are u looking at? ok abt the passmarks... the benchmarkings are all synthetic, u'll find it a nice edition with the 4890 cos i've seen wat a monster it is, and then came the 5 series - they'r makin headlines!
if the PSU's no problem, then go for the 5850 instead, u'l be MUCH happier with it. Sapphire is good.