I just kept adding things until I noticed a difference... well, I haven't really noticed one yet in day to day use, although I'm sure there is a negligible one.
I'm running:
OpenSSH (I keep it disabled when not in use)
SBSettings + plugins
Winterboard
LockInfo + plugins
Backgrounder
Notifier
PDANet (I keep it disabled when not in use)
...all of which run more or less continuously some way or another in the background. And I can easily say that my phone still smokes a stock 3G.
Plus, non-background stuff in Cydia can be great:
xGPS
Quake 3
NES 3
snes4iphone
ScummVM (for old LucasArts games)
MobileTerminal
MxTube
Cyntact (awesome!)
The 3GS has plenty of horsepower for all this stuff.