PCI basicly replaced NuBus, so you can't re-use your card. The upside is you'll end up with a better performing card plugged into a better performing slot.

If you're planning on running multiple monitors you need to pay a lot of attention to system software: I'm not sure on this, but I think newer operating systems (9.5-9.2) tend to support monitor mirroring only right out of the box (I assume you use multiple monitors for the extra screen realestate). I can't tell you for sure though, I could be speaking out of my ass on this one. The thing to do is ask an Apple dealer (bearing in mind that they'll try and sell you a brand new computer).

You may find that the trade in on your old system is scant to nil- You may be better off keeping it. I know one guy who has a G3 upgraded PowerMac 8500 as his main work station and uses his old Quadra to burn CDs. You could always use the case as a means to store drives that won't fit into your main computer, network them for archiving purposes, almost anything that doesn't require a lot of horsepower. Ice T uses his very first Mac (a Mac Classic or something, we're talkign Old School Macintosh here) to type up song lists and what not while newer computers do the dirty work of running Pro Tools.