Is it possible to simply upload hey files that are made for an Atmega8/48/88 to an 168? Specs look pritty similar from wat i have seen, but i am not sure about the inner stuff...
At least in my case it was ok ,0)
I have the reverse question - because a chip like the atmega88 is cheaper than the 328, can a sketch for the 328 be used in the 88? I assume that the 88 is smaller in flash memory and other resources, but as a concept would a small sketch work on the 88? Would the arduino IDE be able to program such a thing if it were put into my duemilanove?
For embedded projects that aren't too big, this might shave some money off, which is why I ask, efficiency in low cost and all that 
The 48/88/168/328 share a common architecture. The differences are the amount of RAM and flash, and the 48 does not support a bootloader.
Once upon a time the Arduino software didn't support the 48 or 88, but I think that has been corrected.
When I used the 48 for a project, I hacked at the core code a bit to get support (you shouldn't need to do this), and used an AVRISPmkII as the programmer (by creating my own boards.txt). After that is was just another arduino board.
-j
Once upon a time the Arduino software didn't support the 48 or 88, but I think that has been corrected.
This correct. I have done a lot of work to support all AVR cpus. Most of these changes are scheduled to be in the next release. Until then, check out http://www.arduino-avr.com/