Basing my opinion on things I have read here and there, and with no solid facts to cite at the moment I would say the dual 1.4 G4 would outperform the 1.6 G5 on most tasks. Especially those tasks that are optimized to take advantage of both processors.
That being said, I really wouldn't upgrade unless you can get the money for the dual 1.8 G5 which is a good deal. If there is a difference it wouldn't be enough of a difference to warrant the extra spending. If you plan to upgrade make sure to check Apple's refurbished section because dual 2.0 G5s go for $2200 when they have them in stock.
Hope this helps...