I have some simple code for this, but it depends on the rest of your hardware. I use a mega with a w5100 ethernet/sd card. I would recommend that what ever arduino you pick the SD card is a must. A lot of people hardcode the RFID tag number into their program and I have found based on the 100 pack of rfid tags I bought that I have probably had 10 fail over the past year. Granted I used a cheap rfid reader and the 100 pack i think was $25, so the costs wasn't to bad.
instead of hard coding the rfid tags that are allowed access. I store them as files on the sd card. by doing this, you can create log files, give certain tags more permissions then others. ie Unlock the door for 15 seconds versus keep the door unlocked.
if you want to keep it on the card a quick google will find tons of examples.