Here are some links for this opensource project if you don't know it yet: Google Code Archive - Long-term storage for Google Code Project Hosting.
Here is the schematic: http://www.flytron.com/pdf/OpenLRS_RX2.pdf
So here are few question: Do i see correctly is he running the atmega328 on 3.3V and 16MHz? What is the purpose of that 3.3V voltage regulator that is on the board?
I would like to use atmega328 dip from ebay, which has uno bootloader already installed...It should work right?
I read that as the regulator should be selected to match the crystal speed.
3.3V fotr 8 MHz
5V for 16/20 MHz
Ebay ATMega - perhaps.
I'd be more comfortable buying from a local source.
In the US: Sparkfun, Adafruit, NKCElectronics, www.gravitech.us, maybe solarbotics.com
The RFM22 will work at 3.3V & 5V
~~http://www.futurlec.com/Radio-433MHZ.shtml~~
That's not the same part 
Do you have a datasheet to the module on the schematic?
Yep I have, it has to run on 3.3V.
Edit: I have found I guy who remade the layout of the boards so that the atmega runs at 5V...I'm in touch with him now..
Can two arduinos communicate through i2c if one is running at 5V and the other at 3.3V?
Yes - use pullup resistors to 3.3V on the SCL/SDA lines.
The master determines the clock speed being used.