After lurking for a while, this question finally motivated me to reister so I can respond.
dustin, the first thing you have to decide is what kind of RFID system you want to use. There's some inexpensive 125 khz modules available from SparkFun and also from Qkits. If you want to get into 13.56 Mhz cards, then the modules are more expensive but the cards are cheaper and can have a longer range. The cheapest module would most likely be from rfidgeek.com. In most cases, you will also need a microcrontroller or a computer to receive the card data from the module and decide what to do with it.
My system used three cardreaders in close proximity so I also had the problem with wanting to shut off the RF pulse that the modules are always emitting when they are not supposed to be active. They have a tendency to de-sense each other if they're closer than a couple feet. The quick and dirty method is to simply use a transistor to drop power to the module. Or, since most of these modules have a reset line, just drop that pin to ground.
Hope this was helpful. Feel free to contact me if you need more information.