It goes in a 5.25" bay and allows you to easily swap out 3.5" SATA drives. You could then have two SATA drives, one drive is your normal OS, and the other drive would have your muddying OS.
There is also virtualization. Not sure if that is an option for you.
The advantage of a second box is that you can use the main and test machines side by side. if you switch drives, you're cut off from your main development environment while you're using the other system.
There are disadvantages too, of course, such as increased desk space, power usage, and heat in your office, and inability to work on the subway.
VMs are good, though VMs don't solve the original problem of letting you test on an older CPU. (Their IO is generally slower, but not in the same way as an older machine.)
http://www.amazon.com/gp/product/B000KS8S9W/ref=as_li_ss_tl?...
It goes in a 5.25" bay and allows you to easily swap out 3.5" SATA drives. You could then have two SATA drives, one drive is your normal OS, and the other drive would have your muddying OS.
There is also virtualization. Not sure if that is an option for you.