I suggest you get a Sapphire 9800np 128mb.
Chances are you'll have Samsung memory on it
So you'll be able to flash the bios with a 9800 pro bios so you'll end up with a 9800 pro that cost you only $249 at newegg. It'll be noticeably faster than a FX5900.
But it has to be a Sapphire since it seems they all have Samsung memory on it. (the 9800nps that is).