virtualbox.org

I'm looking to build a machine that can host the following operating systems:
Windows Server 2008
Windows 7
Windows XP
RHEL 5
RHEL 6

Whats the recommended RAM to support this setup, and what's the best advice for a host OS vs Guest OS? Should the most widely used OS be used as host?

There really is no "right" answer no this question. If you are comfortable using Linux then the linux hosts tend to be a little lighter and faster. Otherwise Windows 7 or 8 should make a sufficient host.

As far as guests, are you planning on running ALL the gusts concurrently? Is so you should get enough RAM for each guests need and probably 4 GB more. Running all this oat once is going to take a lot of CPU, a lot of RAM and a lot of disk I/O and space so MORE and FASTER is the key.

I would personally try and get a server type hardware with a RAID array and gobs of ram, a dual quad core at least. Setting up Windows 8 on this might not be possible, so you're going to have to go with a Windows server or Linux server OS.

This type of question has no right or wrong answer, and it will entierly depend on your own requirements for each guest. Do not expect an answer to this kind of post here.

ok I was going down the right path. Since this is my first trial with the software I wanted to lean on the expertise on this forum for advice. thanks

We are always happy to share our expertise, but here it would be like asking to total strangers what house you should buy - it's just impossible to answer.