I would forget about Sempron as they only have 128 Kb L2 cache.
I think Sempron is the replacemt for the old AMD Duron so it may be high in Mhz but with only 128 Kb cache it will be noticeably slower than the same speed CPU with 512 Kb or above.
The Athlon XP come with 512 Kb.
The Athon 64 socket 754 and the Winchester and Venice core socket 939 all have 512 Kb.
If you are really rich (around £230 inc vat) then you could go for a AMD Athlon 64 3700+ / 2.2 GHz processor socket 939 with 1mb of L2 cache
The Intel P4 Prescotts have 1 mb cache.
You pay your money and you take your chance Jon