I've thought about doing this before in a couple scenarios as well. There are a few things you will need to get. The first is going to be a separate USB keyboard and mouse (if your existing front desk computer doesn't already have a USB keyboard and USB mouse.) Next, you're going to want to get a USB hub that you can plug both your USB mouse and USB keyboard into. Then you need a USB to Ethernet adapter that you can plug that USB hub into. You can then use normal Cat5, Cat5e, or Cat6 ethernet cable to run up to 300 ft. distance to connect your USB hub in the front office to your main computer in the back room. Now you have two separate keyboards and mice connected to control the computer.
The next part is to get your video from your back computer to your front office. You can probably use your existing monitor if you wish to save money, and run a really long VGA cable from the back office computer to your front monitor. There are 100 ft. lengths of VGA available readily online, and you can even purchase greater lengths than that. However, be aware that using a VGA cable greater than about 25 ft. will introduce some lag and slight loss in display quality, so you may have to invest in a VGA signal booster if you have to run the monitor cable a great length.
When we did this exact setup at one office, we purchased a Dell P2211H monitor for the front desk mainly because it had a built-in USB hub right in the monitor so it makes for a very cleaned up and professional look. Additionally, these monitors have both VGA and DVI connectors for whatever cable type you wish to use. Finally, the Dell Professional series of monitors are great quality! They really are a great high quality display and not very expensive at all especially considering all their features. However, if you're planning to do design work off the front monitor as well, you might as well spend a little bit more and the the Ultrasharp series of the Dell monitors as they have a very high quality IPS display panel ideal for design work!
