I do not think that ATMega8 will suffice, nor will the ATMega164 because the other ic's all have hardware USB, and if you want to put in USB software wise you will need quite a bit of coding.
Check pages 18,20,21 HERE for help building your own board. I made a few posts on building some, such as tips and information that I know.
there is a software USB driver for the atmega8, I have used one before for a different project.
Not sure how much memory etc you would need, and If It would all fit.. but it could be possible
http://www.obdev.at/products/vusb/index.html
(Atmega8 usb driver)