Great set-up!
9.2.2 runs like a dream on my "stock" 9600/200MP with a 9200 and 300 and some MB of RAM. IMO, Apple in some ways intentionally "handicapped" these machines by stopping them at at OS 9, as 9.2.2 really seems to open up on them. I've seen this with both the 8600 and 9600.
With some work, you can run Leopard on your set-up, and it should run fairly well. It's more involved than installing Tiger(which you can do with Xpostfacto), but you have everything you need to run it. The 9600 is really a better platform for this than the 8600(which I've been dragging my feet on installing Leopard on) with it having more(max) RAM and more PCI slots.
The 9600 actually has two separate PCI buses, which can be an advantage to minimize bus congestion when you're stuffing stuff into them. Enabling Quartz Extreme in OS X with the 9200 will perk OS X up nicely, and it's not as big of a concern as on other computers since-if you spread your cards out correctly-you have more available bandwidth.
I'd suggest adding USB and FW cards(or a combo card). These can make your life a lot easier when it comes to getting files in and out, and the right cards will be plug and play in both OS 9 and OS X. A 10/100 card is also a nice addition if you're going to get on the internet, as the onboard 10baseT can be painfully slow.
