I've tested the effects of this by suspending using ProcExp. I've also stopped/disabled most services leaving
DCOM Server Process Launcher (some software won't start without this)
Plug and Play
Remote Procedure Call
And when installing, Windows Installer.
For LAN, Workstation and Server.
Some processes need to be closed/killed because suspending them makes Windows UI not receive clicks.
This is fairly useless when not running anywhere near 100% CPU load, but when you do, the effects are clear - things that reacted sluggishly are smooth, audio, video stutters especially the ones that happen rarely once or twice a movie, are gone. This is hardly surprising since the CPU gives the software its full attention. Obviously lower-end machines will benefit more. Before gaming, just pause/kill everything, play, then resume/start. The number of context switches drops from around 5-10K (20-30K max) to around 800-1200.