I'm building a similar rig and was going to use an x1950xt card for it. Then Checkers offered me an xt1900xtx that I'm going to get.
One thing to look at is the timing on your RAM for the P5b board. I couldn't find any Corsair DDR2 800
in 1gb sticks that they recomend, only some Kinsington 512 sticks along with micron and some other maker. Evedently the timing must be either 5-5-5 or 6-6-6 at 1.8v, their recomended makers list was rather small, something you should check out before purchasing.