> You will need cycle-accurate emulation or very nearly so for some games,
Yes for some, but how many? Back to my other thought I had the other day for hacking a portable C64DTV and using something like a SDMMC or compact flash for storing tape/disk images. Then it struck me that the c64 chip in it is so accurate you'd have real loading times. On a battery operated device who wants to suck up 3 minutes of battery for loading times? So you can instantly knock off any games that used multiload & just concentrate on single file'd games. You still get the classics but don't have to go all out with the emulation.
This is all IMO, I've found it more fun to concentrate on getting one game running then move on to the next than spend the whole time coding and only then seeing any results.