At work I'm running about 60 guests across 3 hosts (with RAM and CPU free, I could probably push things up to 90 guests, but really want to upgrade the hosts to dual Six Cores and 64 GB of RAM). Each host has dual Quad Core CPUs, with 32 GB of RAM. The storage is provided over iSCSI (two bonded 1Gb links) by a 24-drive array, organized into three arrays of 8 drives each under RAID6. Each individual drive is 1 TB, so with RAID6 that's 6 TB, or a total of 18 TB (which is way more than enough, because if you have a machine that's trying to eat up disk space as quickly as it can, it's not a good candidate for virtualization).
Once everything is up and running performance is very good. But a cold boot is painful (taking about an hour), even serializing the the loading process still causes so much disk contention that the arrays are saturated for a good while. (We have plenty of battery backup, and a generator, so cold boots are extremely rare).
