For those of you that saw my VPO setup at MakerFaire North Carolina, I have added some links in the sidebar to VPO related sites and resources.
If you want to try out VPO software without having to install software to your hard drive, I would try the jOrganPup distribution which boots from a USB key and runs on most PC hardware. It runs very well on older hardware.
The MidiTizer software is an emulation of a theater pipe organ. It is more complex than jOrgan Pup and is intended to run under Microsoft Windows. There is a version that runs in the WINE Windows emulator under Puppy Linux.
The Hauptwerk software is a commercial product that run on Mac OSX or Windows. There is substantial documentation on line. It will run in a limited "free mode" or with a license key. The free and purchased versions are the same...the USB license key activates the additional functionality.
M-Audio Keystation 61es - this is a semi-weighted keyboard which is often used in DIY organ consoles. I have been buying my 61es keyboards from Unique Squared, an eBay vendor that sells B-Stock keyboards that have minor blemishes (usually on the case and are fully guaranteed) for $124. Since I am removing them from the case, the blemishes are usually irrelevant. Brand new are about $170, so this saves about $50/keyboard. I have ordered 2 and they are both fine.