Voodoo2

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has about 3 times the polygon calculation performance of Voodoo and about twice the fill rate performance of Voodoo.
comes with one Pixelfx2 and two Texelfx2 chips. Voodoo came only with one Texelfx. This enables Voodoo2 to render multiple (practically dual) textures per pixel in one single pass, used in several special game effects. In the past this rendering of multiple textures required multipass ("yeah, Lilou", only for Fifth Element fans), as e.g. in GLQuake.
supports tri-linear filtering without a performance hit, resulting in even better image quality
supports edge anti-aliasing
supports bump-mapping (software only)
Voodoo2 will also only run in full screen mode
one Pixelfx2 has now got enough performance to render in 800x600 fast enough, so that it makes sense using 4 MB frame buffer, enabling resolutions of up to 800x600, however it will be up to the card manufacturer if 2 or 4 MB frame buffer will be used
you can use two Voodoo2 cards in your system, connected with a pass through cable. These two cards can do 'SLI' = Scan Line Interleaving. This is nothing else than that every odd line of the image is rendered by one, the even line is rendered by the other Voodoo2 card. This about doubles the performance and enables resolutions up to 1024x768. SLI has already been used by Quantum3D in their Obsidian boards, based on the old Voodoo chip. The Obsidian 100SB e.g. uses SLI by placing two complete Voodoo units on one board. I guess you are aware of the amazing performance of the Obsidian 100SB. I expect Quantum3D to come out with a high performance Voodoo2 board soon, which will probably use as much memory as possible and also include 2 Voodoo2 units for a one board SLI solution.
there will indeed be an AGP version of Voodoo2 cards, but I guess this is mainly a marketing gag, so that 3Dfx doesn't look too bad regarding AGP technology. The AGP Voodoo2 card can only work with a PCI 2D card, there is no chance of doubling performance via SLI, because this works only with two PCI cards, Voodoo2 doesn't support any real AGP features as side band addressing, DiME, 2x, ... instead of this it's only taking advantage of the higher data bandwidth of the AGP port. 3Dfx calls this 'PCI 66 Mode', because of the AGP's higher clock rate of 66 MHz. The AGP card can be faster than one PCI card, as long as data bandwidth is the bottleneck in a game. Two PCI cards in SLI will definitely be faster than one AGP card though.
Voodoo2 cards can come in several different memory configurations, a good way of finding out if the card manufacturer is only interested in making some quick money by using as few RAM as possible. As already said above, Voodoo2 can use up to 4 MB frame buffer, offering resolutions of up to 800x600 when Z-buffer is used. You also can equip it with only 2 MB frame buffer, limiting your resolution to a max of 640x480. The same story is valid for the texture memory. Each Texelfx2 chip can either use 2 or 4 MB of memory. This gives the following possible configurations: 2-2-2, 4-2-2, 2-4-4, 4-4-4
Diamond's Monster 3D II will offer resolutions of up to 800x600, coming in 4-2-2 configuration = 8 MB onboard RAM.
Voodoo2 will run at 90 MHz by default, it uses 25ns EDO RAM
Voodoo2 will be less CPU dependent, as we will see in my benchmarks, but it scales a lot better than Voodoo. This means that it's always faster than Voodoo, at a lower percentage in systems with a weak FPU, at a very high percentage in systems with a powerful FPU.
D3D games will all be compatible with Voodoo2, Glide games are also supposed to run just as well on Voodoo2 cards, but we'll see if there won't be some compatibility issues with some games in the beginning.
Voodoo2 is so much faster than any other 3D chip, that some games will include a special version for the high performance of Voodoo2.

As already said, Diamond's Monster 3D II will come with 8 MB 25ns EDO RAM, supporting up to 800x600 3D resolution with 16 bit Z-buffering. This makes it already better than the announced products of some of its competitors, which are only using 6 MB RAM, hence castrating the card to a max. resolution of only 640x480, although it has well enough performance for 800x600. Diamond will offer two cards running in SLI mode, which Diamond calls MegaMonsterTM.