Socket A is the same thing as a socket 462. Athlon XP processors fell into the Socket 462 catagory. I think they made those up to ~ XP 3000+. I would not hesitate to buy one of those suckers for an upgrade. I also think they made semprons for the budget seeking upgrader. There's room to make that processor about 2x faster than it is. I'd recommend an XP 3000ish
You said something about an M processor, leading me to think you're using a laptop? If so, here's a 2600 M
If you're not looking for something in the M department, here's an XP 2800+
Just my thoughts on the subject...