ok here it goes...
athlon slot A - first ever athlon processors built... old as hell don't bother
Athlon TB - athlon thunderbird... it's fairly recent but dont' bother, still too old
only goes up to about 1.4 ghz anyways
Athlon MP - Athlon's server series designed to compete with Intel Xeon series processors, don't bother unless you want a server or dual processors... but I'm pretty sure yours is for home, so don't worry about this
Duron - There are two cores... the first one I don't know what it's called but it's a modified TB core with less cache I believe
The new Morgan core has more SSE instructions and crap so it's as good as a P3 or even a ThunderBird... it'll compete well anyways... a "budget" idea would be these Durons, so I like 'em. (they are not the equivalent to a celeron, built to compete with them, but they're way better)
AMD XP - These are the gems... Amd's top of the line processors RIGHT NOW. They are soon going to be replaced by the thoroughbred cores and are going to replace the Duron line of processors. Yes, the durons are done for.
Thoroughbred & Opteron- These are the new generation of processors built on the .13 micron core I think... they should be fast and they're coming out soon, so if you want the best wait for these motha's
Sledgehammer and opteron are just other names for the thoroughbred... I think the sledgehammer will be for retail sales while opteron for Dualies and servers? not sure so don't quote me on that
But if you were to buy a computer right now!
AMD XP FOR SURE!