Yep virtualbox is probably your best bet for the 'free is best' price!
I've used it on a few different machines - although I'm definitely not impressed with it's performance running Windows VMs on an OSX host - however, that's not your problem so it's a non-issue.
Your other options (also free so you could try all 3) are Microsoft Virtual PC 2007 also Virtual Server 2007 - however, I'm not sure it will allow install on a workstation-class OS. Also VMware Player is a very compatible and reasonably quick VM solution - although as it's name implies it's not capable of creating VMs from scratch. There are literally hundreds of 'appliance' VMs available for it however, so it might not even matter depending on what servers you are testing (lots if you mean Linux servers - or none, if you mean Windows... legally at least).
If you decided you liked VMware Player - there is a legal way around the limitation if you are so inclined. OK... maybe not entirely
honest - but legal nonetheless. There is a free 30-day trial on the full version of VMware Workstation - which allows you to do pretty much everything. If you have all of your source disks, etc. prepared ahead of time, 30 days should be enough time to create enough VM's to choke a system completely! Just set them all up during the trial and then use Player from that time on to interact with them.
Player will allow software installs, snapshots, reverting, importing, etc... just not creation of a new VM from scratch.
I use both virtualbox and VMWare Workstation (and Fusion on my wife's Mac so she can run Win7 without having to reboot constantly). Both are pretty straightforward and well supported.
And when you get past the initial stages and want to really have fun - provided you have an extra PC or want to build a compatible white-box server (lots of informtaion on their forums) - I highly recommend VMware ESXi as a hypervisor. I just completed virtualizing my entire datacenter in my office and if nothing else - the difference in fan noise between 10 rackmount servers and 1 massive tower server is heaven to my ears. Of course, it's heaven to my company's bank account too since the A/C bill dropped by almost $200/mo!