The Thunderbird has been one of the most talked about processors cores simply because it was thought to be exactly what AMD needed in order to regain the performance advantage over the Pentium III.
The Thunderbird, like the Duron, will take the current Athlon and move its L2 cache on-die. For cost and performance reasons, the Thunderbird core will feature a total of 256KB L2 cache running at clock speed versus the 512KB of L2 cache running at between 1/2 and 1/3 clock speed on the current Athlons (K75 core). Combine this with the Athlon's 128KB L1 cache and AMD once again has the cache advantage over Intel.
Since the Thunderbird will be moving the L2 cache on-die, there is no longer a need for the Slot-A processor card and we will thus see the Thunderbird available in a 426-pin Socket-A package.
The biggest disappointment here is that, although we all remember this slide below indicating that the Thunderbird would be available in a Slot-A package as well as a Socket-A package, AMD wants to move to Socket-A very quickly and will only be supplying OEMs with Slot-A Thunderbirds.
According to AMD, you shouldn't be able to go out and purchase a Slot-A Thunderbird, meaning that you will most likely need to get a new Socket-A motherboard for use with the Thunderbird. While it is inevitable that there will be some Slot-A Thunderbirds leaked from the OEM channels to on-line vendors, for the most part you can expect the Thunderbird to be a Socket-A solution only.
AMD is pretty far behind with the transition to the cheaper Socket-A packaging for their processors and they want to make the move as quickly as possible. If you remember, Intel took quite a while to make the transition to FC-PGA 370, and even today, you can find a good number of Slot-1 Pentium IIIs. Since AMD didn't start the transition at the end of last year like Intel, they have to act very quickly in making this move. This will definitely upset the many Athlon users with Slot-A motherboards that will be relatively useless when the Thunderbird hits the streets this June.
Upon its release, the Thunderbird will replace the K75 core as the Athlon processor, so when June rolls around and the new Thunderbird core is shipping in volume, the processors that the Thunderbird core will be found in will still be called the Athlon. The way you'll differentiate between the new Athlon and the old Athlon will be because only the new Athlon (Thunderbird) will be available in a Socket-A package while the older Athlon (K75) will be solely a Slot-A part.
The performance of the new Athlon (Thunderbird) will definitely be greater than the Pentium III (Coppermine) on a clock for clock basis, although it may not be able to claim the same against the upcoming Willamette.