Powerlogix announced dual G4 upgrades, though it looks like they may be backordered currently....
I would think the dual 7457 is the ultimate way to go, and would wait for one of those.
that said, I have a dual 800 QS, and when it comes down to it, think that selling the current box, then adding the money to it you would pay for such a top end upgrade, is what you should compare to a new/refurb G5 price, then decide if that teeny difference isn't worth the additional $ for AGP 8X, PCI-x, 64-bit G5 future, etc...
I am not one to say wait for the next generation as a knee-jerk response, but whether I was getting a dual g4 upgrade, or a g5 refurb, I'd wait until the next gen is out....
The g5 is likely to further squeeze upgrade manufacturers for dual g4s, who's going to spend really big cash for upgrading a dual g4?? Either they need to get cheaper dual G4 upgrades or it will be hard to justify for anyone with an existing dual.
In fact, apart from running the latest 3-D FPS games, I love my dual 800. Apart from a video card upgrade, my concession to modern gaming, I really couldn't ask/or need more speed for Office productivity, iTunes, movies, web surfing, etc....and as a casual user couldn't justify spending for the upgrade/new machine so I can have a better game machine....yet...