I wanted to contribute to this thread as well.
I found that BI5 (and BI4) does not require that much of HW to run.
I have 10 HD cameras streaming to BI (those cheap Chinese cameras).
I used to run BI on dedicated PC, but since i do have VM host available i decided to give a chance for BI to run on Hyper-V VM as well.
... one host less to care
VM host is HP DL380p G8, 2 x E5-2630v2, 128 GB RAM (16 x 8GB/PC3-10600), 6 x 2TB 7200rpm SAS HDD's on RAID6, P410i w 1GB RAID
Not a brand new hardware nor the most powerful.
There is few VM's running on host.
BI VM has 8 cores, 4 - 8 GB dynamic RAM, 2TB virtual disk for recordings (about 30 days motion detection recordings)
BI VM CPU's has constant 10% load and RAM is required 3.5 GB (64bit windows).
The VM is dedicated to BI, so there is no additional services running.
BI is used via IU3 and VM maintenance is done using RDP (with NLA of cource).
I could even cut some cores off if i get short of resources one day.