the standard now for pcs is usb2 if a pc still has only usb1.1 it is a very old stock one. As to the chip if you have decided on AMD your best bet is the socket 939 as the socket 754 appears to be the one that is now being replaced. Sempron is the cut down cheap equivalent to the celeron. Go for the best chip you can afford.
Without bringing the dual core chips into the conversation the Athlon 64 is roughly equivalent to the Pentium 4 and the Sempron is roughly equivalent to the Celeron. The latter 2 are the budget chips from AMD and Intel. So to answer your question you would compare a Athlon to a Pentium 4 its not fair to compare either to a Sempron as its a budget chip and its a bit slower.
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