I've had real difficulty running SheepShaver on a non-dual-boot PPC. It's not even "native" on Intel, so I imagine results would be worse. Fairly old games, some 3D (Mechwarrior 2, Total Annihilation) worked only a little before crashing. I second the idea that it can't handle games. I also second the idea of getting an old Mac. If you have a monitor, you can probably pick up a beige or even B&W G3 (not iMac) for under $30.
EDIT: Should add that, at least according to the SheepShaver FAQ, you need System 8.5 or higher for Mac; it's only Linux, for whatever reason, that can run 7.5.5. System 8 and above, last I checked, are NOT freely available from Apple; you'd need a disc to do it (think I used a System 8.6 restore from an iMac)
"In Mac OS X: PowerPC MacOS 8.5 up to MacOS 9.0.4 "