Have you tried the Atmel web site...?
There's only a few variants that are important. They are (classed by number of pins):
Tiny85 - 8 pins
Tiny84 - 14 pins
Tiny4313 - 20 pins
Mega328 - 28 pins (Arduino Uno)
Mega2560 - 64 pins (Arduino Mega)
There's variations of each with different amount of memory but they're not necessarily cheaper on the street because they have less memory.
eg. The Tiny85 family has Tiny85/Tiny45/Tiny25 with 8K/4K/2K of program memory. The first digit in the number is the amount of memory, the second is the family.