A quad core system is less expensive than 4 single core systems: drifterlee mentions a quad system for $1500. And you'll only need a single OS license (included) plus a single Poser license.

But a quad core system also performs less than 4 single core systems combined - provided that the single core systems have enough RAM and disk to work with. 

AMD dual core systems aren't that expensive anymore ($900 for a decent base system), so maybe two AMD dual core systems might be the way to go...

As far as running more than one render on a single computer - not right away, but you can do it using VMware. The advantage of VMware over two physical machines is that your Windows license is valid for both the host machine and all virtual machines on this particular host. Plus, VMWare is free. You'll probably need a Poser license for each VMware instahce however.

VMware becomes a great option when you have a machine with more than 4 cores, for example dual Xeon quad core system (8 cores). Make sure you have lots of RAM on such a machine. VMWare recognizes all installed RAM and will divide it over the hosted virtual machines.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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