You could also extend a second monitor/touchsceen from your existing "Touchscreen server" with a USB-->VGA adapter and a USB over UTP extender. Ideally, you could have several of these units driven from one "server". Your interface software would have to launch and re-launch at specific coordinates to keep them on the proper monitors and only one screen could be touched at ant one instant, but other than that it could be a cheap way to provide multiple touch screens plus you only have to worry with one PC driving them.
I have successfully tested this with the HouseBot swremote and 3 local monitors. In HouseBot it takes a vbscript and some registry manipulation to place the software remotes at the proper coordinates, but it works well on the test bench. My particular application adds an additional layer of complexity as I am running the swremotes in a Wine session under Linux that you wouldn't have unless you wanted to avoid another XP install and your software will run under Wine.
Lastly, if you choose to go with a 1 to 1 PC/touchscreen; check out geeks.com for refurbs, or better yet; ebay, if your house is like mine you probably have plenty of hiding places for PCs withing 10-15 feet of your desired touch location (directly above/below in the attic/basement). Instead of a mini PC, you can also get cheap older laptops from ebay to use. Keep the laptop closed and it hides under the lowest couch or chair without a problem - that works great for touchscreens sitting on coffee tables of side tables...
Terry