I just picked up a 5,1 (for £330) as id maxed out my 3,1... Ive put a pair of X5675 xeons in it that I had spare from a R510 server I am playing with, ive transferred across my old 970GTX, its got 24gb RAM, 4 port USB 3 card, pair of 256GB SSDs and a pair of 4TB HDDs.
All in all more than capable of everything I currently need, Im planning on replacing the 970GTX with a 1080GTX when the price gets a little better, along with a RIFT, and also thinking about a PCI-E M.2 adapter to add a nice faster bootdrive SSD. It will take a pair of X5690 CPUS which I will probably buy at some point simply to max its capacity.