I too have been doing a bit of research into MIPS based systems and the short answer is there are no real PC like MIPS boards out there. The closest are the NetLogic ATX hardware dev kits for their XLR processor. The XLR has four 10GBE ports and quad channel memory as well as a hypertransport interface. Its possible one could engineer a motherboard using the XLR with a HT-PCIe bridge for I/O and/or possibly use an AMD southbridge. The problem is the Video card BIOS code is expected to run on an x86 CPU so any video card in that system would need to have a MIPS BIOS. Same goes for bootable storage cards (SCSI, SAS, SATA etc.). The XLR is pretty damn sweet, up to 8 cores, each 4 way threaded (like the Sun UltraSPARC T1).
As of 2009 there is a 1GHz Loongson quad core but I don't know of any ATX motherboards based on it. The future Loongson CPU is supposedly going to be a "4+4" 8 core MIPS64 processor with a clock speed of 1GHz. An there will be a 1.5GHz+ 16 core in 2012 (supposedly). They all have Hypertransport which is like NUMALink so building an SSI system with over 100 cores would not be a problem

