Well, that replacement is due at the end of 2002 or beginning of 2003. The successor to the Athlon XP die is the Thoroughbred which is just a die shrink from .18mm to .13mm. It allows for faster speeds. No architectural changes/enhancements are in it, just a die shrink. May be better for overclocking than the current crop. It will be due in May/June.
The ClawHammer is where it's at. ClawHammer, the workstation/high-end PC will have the capibility of both 64-bit and 32-bit processing power. It will be called the AMD Athlon XP-64. Many (including myself) will be awaiting this beauty. All other PC-Related stuff put off on hold for this one. If it fails, AMD will die. It won't, though, which is good. The SledgeHammer, just named the Opteron, is for server and other data-intensive stuff. Microsoft will be putting in much $$$ for the Hammer series, it's sure to be very revolutioniary! All they need now is some 64-bit programs...
As noted above, it is due out at the end of '02 / beginning of '03. I won't go into the specifics, as the links below provide ample info.
AnandTech Review
Tom's Hardware Review
AMD Zone Links (most complete)