--This unit will read RFID tags and communicate the data to a website via a GSM module.
--It will simultaneously communicate via Xbee with another Arduino with an LCD screen.
There are different types of RFID reader out there some even dn't need power to operate and some need.
You have to be clear about what type of LCD you would be using because there HD44780 driver based character LCD's and there are Graphic LCD's which will eat all the pin's on your arduino and there are also sparkfun nokia 6610 breakout module which will use less of arduino pin's and there are some printer LCD's that use ONLY 3 PINS to driver the LCD.
i would suggest use a mega for the one with which a LCD would be used and use UNO for other stuff