if you're concerned with your budget, go for highest capacity dimms (2x8 over 4x4), and fill up the dimms later
if you haven't, buy& allocate separate drives(RAM Drives are really nice, SSDs preferable, HDDs) for each running VM
I believe 2500k maxes out at 32GB, so 2x8 seems ideal (if you have 4 dimms lots). Dno't think you can put 64GB ram on sandybridge, could be wrong
2500k doesn't support VT-d. might want to look at a non-K or xeon chip for VT-d support, might be able to reuse motherboard
(VT-d useful for direct PCI/SATA passthrough. if you don't know what it is, probably not useful for you, just keep your 2500k)