About CPU uppgrade... the best one available is the CSPPC233 w. 060/60. If we're talking future possibilities with might never arrive there's the Shark/Shark+ from Elbox, no Dragon for the A4000 have been mentioned yet afaik, there's also been rumours about a new G4 PPC card in the making for the A4000.
I'd strongly suggest you get a real Voodoo3 2000/3000 card as the Banshee is missing the 3D capabilities and thus Warp3D won't work with it. And unless you get a scandoubler working for it and have a switchbox (or device) to switch between gfx card and scandubler you still need 2 monitors.