It could be that the card itself is incompatible with the sempron processor.
I'm using a sempron 31000+ processor on a abit kv8 pro board (saving for the amd 64 processor which this board will handle has well) I've tried both the fx 5500 and the fx 5700 to no avail. Both graphic cards were unstable blanking out the screen what ever I tried, mobo driver/graphic driver omega drivers, all had the same result. I did put my fx 5700 into my other computer P4 2ghz asusus p4b266 mobo, and it worked perfectly no problems at all.
Out come I took it back exchanged for a radeon 9550 vguru (abit card) and have no problems, and in IMHO, it is a far better card altogether, and very over clockable.
I hope this helps a little bit, at least it might give you some pointers!