All ATmegas are basically the same in how they operate, in terms of the user.
Of course the pinout will be different, and you will have to modify the source code a bit, but yea the hardware I have on the $50 Robot is the bare minimum you will need for any ATmega.
I didnt check the atmega8515 datasheet, but if you can find equivenlent pins on it, then the same hardware will work.