Well, since we have our own forum now *gasp!*, I figured I'd post my impressions on my choice of motherboard in my dual 64-bit system.
By and large the K8T Master2 is the cheapest board for dual Socket 940. However, there are a few things you need to be aware of if you're considering purchasing one of these.
1) The cooling situation on this board is downright bizarre. It does not take standard Opteron heatsinks. The board has a motherboard socket hole pattern matching that of the Intel Socket603/604 Xeon and Socket423 Pentium IV. However, at least one processor socket will have a plastic HSRM that accepts Socket478 Pentium IV heatsinks. I say at least one because some people have these on both sockets, though mine only had one on the top CPU socket.
MSI bundles heatsinks with the board, but they are loud unless you use the SmartFan control feature in the BIOS. This feature cuts the voltage to the CPU fans until the CPU temperature rises above a minimum threshold temperature that you set (60 C and up in increments of 10 C) at which point it brings the fans up to full speed. I found these heatsinks to be inadequate for use on two Opteron 248's in my Lian Li PC-7 midtower.
I use a pair of Swiftech MCX478+ heatsinks with a modified retention brackets (the $3 extra ones) coupled to high-output 80mm Panaflo fans. I modified the retention brackets by cutting off the holes that allow mounting on Socket478 systems, leaving only the Socket423/603 holes and allowing me to use 80mm fans. Alternatively you can buy the motherboard retention kit for a MCX603 direct from Swiftech; it bolts onto the MCX478+ without any modification required since the MCX603 and MCX478+ are essentially the same heatsink.
Behind each CPU socket on the back of the board is a massive steel load plate attached to the board with insulating foam. The load plate has threaded inserts that come into the holes surrounding the CPU sockets. The threads used are metric 3mm threads. If the heatsink you're planning to use has English threads on it, you will need to either replace the offending bolts or standoffs with 3mm metric ones or cut 3mm metric threads onto the bolt/standoff. I chose to cut 3mm threads onto my Swiftech standoffs using a thread die you can get at a decent hardware store.
MSI's explanation for the odd hole spacing is so that you can easily watercool it by not having to wait for Opteron and Athlon64 water blocks to become available, but rather use existing Xeon blocks.
2) There is about 2mm of clearance between the AGP Pro slot and the lower CPU heatsink. Don't even think about installing a Zalman heatpipe cooler; it won't fit. The stock MSI cooler for the lower heatsink is offset from the center of the CPU socket to allow extra room for your AGP card at the cost of decent cooling. Depending on the memory heatspreader configuration on your graphics card, you may not have room for an after-market cooler. Mine doesn't have memory heatspreaders, and I live with having only 2mm clearance between my Panaflo and the AGP card. The card hasn't complained yet.
3) Many people have had trouble using Corsair registered ECC DDR400 with their K8T Master2 systems. Their machines exhibit stability problems and occasional pagefaults. I had these problems initially, but they went away when I bumped the DDR voltage up to 2.65V. My machine has successfully complete 7 passes of Memtest86+.
4) Don't use Memtest86 3.0. It cannot properly detect AMD64 CPUs and chipsets. Use Memtest86+ instead.
5) The two IDE connectors, floppy disk connector, and both SATA headers are in the bottom front corner of the board. Make sure cables meant to reach drives in the top of your case are the same length as your case's height. Really.
6) The K8T Master2 does not support dual floppy drives. :(
7) Since this board is a standard ATX board (12"x10") and not an E-ATX board (12"x13"), it will fit in most standard ATX mid-towers and desktops. Just make sure you have adequate ventilation to dissipate two fast, modern processors worth of heat.
8) Due to space limitations and less confusion-related problems with people running a single Opteron or AthlonFX on this board, there are only two banks of RAM (4 slots). They're both controlled by the top CPU, so you there aren't dual memory interfaces on this board. In other words, if the second CPU has to do something in RAM, it has to go through the HyperTransport link to the first CPU and use the first CPU's memory controller for memory access. This isn't any slower than the CPU having to go through the Northbridge for memory access like in a normal 32-bit system so you aren't likely to notice unless you're benching versus someone using a 4-way Opteron system.
All that being said, this is an excellent performer and I'm satisfied with my purchase. It's about half the price of the next board up, and is the only dual processor board that I'm aware of that uses the VIA K8T800 chipset instead of the AMD 8000-series logic. The AGP performance on the K8T800 is supposed to be better, but I haven't really looked into it.