My two cents: a Voodoo3 3000 on a Prometheus is the fastest "ZIII card" out there (kudos to redrumloa for figuring out how to fit one in the A3000 desktop case).
As for using the PIV in a desktop A4000 with a Toaster, you can but not both in-line with the video slot because there is only one in the standard busboard. There are several tower replacement busboards that give you two video slots, so towerizing the A4000D motherboard and getting one of these would be best bet.
PIV's seem to go for ~$200US on eBay lately, iirc (seems a fair price really because you're getting a great FF/SD and a 4MB graphics card in one).