Going Virtual with VMWare ESXi

I have a bunch of low end servers running many different things. I never like to keep all my eggs in one basket, so I spread services and apps over a lot of servers. I need a better disaster recover plan so I have decided to virtualize as many servers that I can. I do not have any money to spend, expect maybe for hardware. I think I am going with the free version of VMware ESXi. It's free with lots of community support. I already have one server running Ubuntu with 3 servers running on Virtualbox. It has a quad core Phenom 9600 with 4 GB of ram. I have a SMTP, ASSP spam filter and Blackberry server running on it. It runs really smooth and has never crashed. I really like Virtualbox, easy as pie to setup and runs like butter. It just doesn't have a lot of admin type tools and "live" backup or migration tools. I may still keep the Virtualbox server up for a while to see which I like better.