[G4] Single or dual G4 upgrade for QS867?

[Hal]

I'm considering upgrading a Quicksilver 867mHz G4. I've been looking at 
OWC's 1.5gHz and also at the various dual CPU options (Giga and 
Sonnet). I prefer an adjustable upgrade, so I can tweak it for maximum 
performance.

Would I see more of an improvement from a dual 1.33 or a single 1.5? 
I'm mostly playing games, some iMovie and iDVD, some light photoshop 
work and VPC.

Any info is appreciated.

Thanks in advance.
-Hal