On the bottom right of the Virtualbox window, there will be a small CD icon. Right-click on the CD icon, and switch the virtual drive from your copy of i Boot to your actual DVD drive (with the Snow Leopard DVD in it).
Then press "F5" to refresh the i Boot menu, so that it can detect the new installation disk.
Once the settings open up, go to "System" and uncheck the "Enable EFI" box.
This is by far the most important single setting that you will need to change.
You also need about 10 GB of unused hard drive space.
Right click on "My Computer" on your desktop and click "Properties" to check the stats on your computer.
If you need help deciding whether to make that leap of faith, then you can try installing Mac OS X Snow Leopard on Virtualbox, instead.
Your new virtual machine will show up on the left column of the Virtualbox start page.
Select your Mac OS X virtual machine (single-click) from the main page of Virtualbox, and open up the virtual machine settings.
VDI is the original format for Virtualbox, while VDMK is the format used by VMWare.
If you're considering getting a copy of VMWare, you might want to choose VDMK. I recommend creating a dynamically expanding disk; the only other option, fixed-size storage, will eat up your hard drive. But don't stop now--you still need to change a few settings before your machine will actually work.