Keep in mind with AMD processors, they will be slower than the Xeons except in 80-bit floating point, which is very rare to encounter in recent games. While the AMD Athlon MP is capped at 2600+, the Xeon goes up to 3.06 GHz, with 3.2 GHz in the pipes. Plus, the AMD system will have less memory bandwith. The short of the long of this paragraph is that, in price/performance, Intel beats AMD in the multiprocessor arena with the one caveat being that you must get into the Xeon pricerange first (about $2000 for the box).
Onto compatibility, the Hercules Digifire 7.1 does both work well and actually performs better than the Audigy 2 in most scenarios, plus is tons and tons more stable even in uniprocessor rigs. Oh, you get two FireWire ports on the back too 
For the ATi cards, I have heard of no problems with the 760MP or 760MPX chipsets whatsoever. Or for that matter with the i860 or E7505 chipsets.
Onto boards. The original AMD dual processor motherboard was the Tyan Thunder K7. Onboard this beast comes U160 SCSI, an 8 MB PCI graphics solution, AGP Pro 4X, angled DIMM slots for rackmount installation, and no less than five 64 bit 33 MHz PCI slots, plus dual onboard 3c920 server NICs. As the first board on the market, it had to survive as the only board on the market for a while, so it has all of the features + the kitchen sink. Quite the board, it goes for about $300 now. Since it was the first and no power supplies were available to power it, Tyan invented their own standard, and used it on this and the K7X. You can find adapters for EPS12V to this standard, as well as the power supplies.
Tyan then released the Tiger MP, and it became the second board on the market. Sporting nothing but the usual onboard, plus four 64 bit 33 MHz PCI slots in addition to two 32 bit 33 MHz PCI slots, it fit into the lower end of the market. Current price is around $200.
Later, AMD released the 760MPX chipset, and along with it came the goodness of 66 MHz 64 bit PCI slots with a whopping 533 MB/s of bandwith, double the old 266 MB/s of bandwith the previous 760MP solution, and quadruple that of standard 32 bit 33 MHz PCI. The world+dog waited anxiously for this … and then it shipped with broken USB. The good news is that the broken USB has long since been fixed, and we now have 66 MHz PCI slots for the AMD 760MPX board. This time, however, not just Tyan was the launch partner. AMD had several other companies, although most of them are EOLing their board, leaving Tyan as the only player in the market actually making boards, although you can still find others (just do a search at the “usual suspects” motherboard makers, less SuperMicro who is an Intel-only shop). To that end, I’ll go through the Tyan boards both for briefness and because my fingers are getting tired.
Tyan released first, although not by too much, the Thunder K7X. At around $325, the K7X again packs as much whallop as its Thunder K7 brother, but then pours it on a bit more. Same dual NIC configuration, same angled DIMM slots, the same dual channel U320 SCSI, and the same memory configuration, but all hooked to a 66 MHz 64 bit PCI bus for maximum performance, plus two slots on that same bus and then four 32 bit 33 MHz PCI slots rounds out the board. Oh, all other features that were on the ThunderK7 are here too, like the onboard video + AGP Pro.
To answer the lower-cost market, Tyan then released the Tiger MPX. Again, nothing onboard but it does sport two 66 MHz 64 bit PCI slots for high-bandwith goodness. About $200.
Ahh, the Thunder K7X Pro. The newest board out of Tyan, it’s an attempt to make the MPX look like a recent chipset by throwing that utility sink in the basement in addition to the kitchen sink on it. Common to Tyan’s Thunder K7 line is the angled DIMM slots, AGP Pro and et cetera, but this one adds support for the EPS12V power supply (finally, a standard power supply connector), Ultra 320 SCSI, one 10/100 Intel NIC plus one 10/100/1000 Intel NIC onboard, two 66 MHz 64 bit PCI slots and onboard graphics. About $475, but a sweet board.
Next installment: Intel boards.
