I'd recommend you fold SMP work units, but not bigadv (big advanced), and just close the FAH client when you want to play the APB (a shooter MMORPG).
How FAH works is, it uses the cpu only when your system seems unused. Say you're keyboarding a question onto the forum. FAH would then work:
1) with the cores that you aren't using, at that moment
2) with the same core you are using, between keystrokes (if you keyboard slow enough), and when you are thinking about what to say, but not hitting keys atm.
It takes FAH about 1/3rd of a second to stop, once it detects that you are using the core that it's working with. For light gaming, it's OK to leave it folding, but for games that require fast reactions and lots of action for the cpu, you'll want to stop FAH while you game. The busier your computer with other programs, the more you'll experience this lag, because your system will take longer to process FAH's requests to back off.
If you APB game is single threaded, then FAH can run at the same time you are gaming, no problem. If APB is multi-threaded, and uses all the cores as you play, then it's best to shut FAH off while you game.