Whistler will appear in two main versions, personal and professional (the latter probably split into workstation, server and data center). The personal version will replace the win9x series, professional replaces NT/2000.
It is not that great a leap from the current 2000 release (it advertises itself as NT5.1), however the UI is greatly different, being much prettier;-)
The convergence between win9x and NT is almost complete with the release of DX8 for 9x(Me) and 2000, I hardly ever fire up Me now. Gaming is well supported in 2000, without the instability of win9x(Me).
I just hope Micro$oft get it right this time, the networking has been very flakey so far, and the latest beta won't even install on my machine (crashes before bootlogging can start!) :-(