'unlocking' a Sempron to AthlonXP is not a big thing since Sempron socket A is actually AthlonXP, just that it's differently rated in term of speed.

in this case, somehow I think the board is the limitation. probably the bus speed got 'haywire' since if it is not 'locked' or manually set to the correct speed (agp speed should be 66mhz only, or 67mhz under very high FSB).