This really belongs in the SFF section, and you could try
Sudhian forums. Anyways, IIRC the Athlon XP is clocked faster than the Sempron at any given rating (eg AXP 2500+ vs Sempron 2500+). AMD simply revised the ratings, effectively lowing the clock speed across the original AXPs. Other than that they should be the same core, unless you're getting a very old Athlon XP.
Shuttle has poor after-sales support for techinical issues. You're best source of help is likely to be from the public forum linked above. However, if you need to RMA they will do it, though in terms of service YMMV. I'm not sure if there are any glaring problems with the SK43G; I suspect it should be almost bug-free. You should check at Sudhian forums though