Max Synch for Strobes...

Q1: For 20D, what is the maximum synch speed for the White Lightning strobes?
Q2: For high key pictures, what is the correct value for the background lights?

Both question requires more info. Mostly distance from strobe to subject.

A1: I'ld say somewhere around 1/125s. That's what makes digital so easy: Give it a try!
A2: If by High key you mean a (slightly) "overexposed" (overall) image, I'ld say that you set the background lights about the same as with a "normal" image. When using a white background, set it to 'just (not) blowing out'.
BTW: I don't think there is such a thing as a 'correct' setting. It's all a matter of taste & personal preference....

I have been using the 1/125 for white lightning, but was not sure if I can increase the shutter speed?

My rebel syncs at 320 for some reason with manual strobes.

Cheers