Here's a link to the unRAID hardware compatibility list:http://www.lime-technology.com/wiki/index.php/Hardware_Compatibility
I recently rebuilt my unRAID server and replaced an aging Gigabyte motherboard that suffered from the HPA bug with a new AMD micro-ATX FM1 board and CPU. I also upgraded the onboard LAN and replaced it with an Intel EXPI9301CT PCI-e gigabit NIC. I highly recommend the LAN upgrade to anyone with an unRAID server. I believe my total cost for the upgrade, including motherboard, CPU, RAM, and NIC, was about $150.
Like duff99 said, try the hardware you have with the trial version of unRAID and see if it works. If not, there are lots of other motherboards you can use instead. Check the hardware forums in the unRAID server forums
and see if anyone there has tried your motherboard. I picked my FM1 board because it was on sale, as was the CPU. I just ran a search in the forum for the board and found several posts where others had used it successfully. I can potentially run about 22-24 drives using this board. It has six onboard SATA ports and enough PCI-e slots to install two Supermicro 8-port SATA controllers and the Intel NIC. If I didn't install the NIC I also have a dual-port PCI-e SATA controller card for a PCI-e X1 slot.
As for the PSU, you'll need to calculate the actual power usage for the drives you want to use along with the CPU and miscellaneous components. It never hurts to get more PSU than you currently need because you'll probably want to expand the server at some point and will need the extra power to support it. I'm using an Antec Nine Hundred tower case that I converted to hold fifteen drives using three 5-in-3 SATA backplanes. I've added another backplane externally for a total of 20 drives. I'm currently using a Corsair 850-watt PSU, recently upgraded from a 650-watt model.