OK... I'll go for it. If money was no issue on buying a single machine, I would buy the 8 core system, stack it with RAM, load it with VMware ESX and dedicate 1 core and at least 2 GB of RAM each to the VMs and run a slew of OSs.
Of course if money was no issue, then why stop at 1 server? Why not built out a datacenter with enough servers and networking equipment to duplicate an entire corporate network, development lab and a hacking lab. I would also use VMware, so if I destroyed a machine, I could revert to an original image and play some more.
Then again it depends on what you want to do. Do you want it solely for hacking individual systems or do you want it to study infrastructure security with routers, firewalls, IDS/IPS, management servers, DNS, DHCP, VPN, wireless, AD, etc. etc.
I could go on, but I'll let others chime in,
CISSP, MCSE, CSTA, Security+ SME