I think the SIIG JU-2NE012 card might be a good choice. It combine USB 2.0, Firewire and 10/100 ethernet on one card. There are a couple on Ebay for ~$15 at the moment. The JU-2NG012 or JU-2NG011 would be a better choice, as they have gigabit ethernet, but I have not been able to find any of those available for a very long time.
Also, I've never tried the JU-2NE012 so I can not say for certain that the three chipsets on-board work well on the Mac. The three on the gigabit versions do, but I have not tried the 10/100 version. As long as the USB chipset is by VIA you're probably okay...
I think the ideal set of cards for a 7500, absent some special need, would be something like an Acard AEC-6820M (ATA-133 support for four drives), the aforementioned JU-2NG012 providing USB 2.0 (or 1.0 under OS9), Firewire and gigabit ethernet, and an ATI Radeon 7000 or maybe a 9200.
Add an 800 MHz or 1 GHz PowerLogix G3 card, and that's about as fast as the hardware can be made to go...
In the photos on Ebay I can see a RealTek 8139 for the ethernet chip (gigabit version uses 8169) and a TI T43AB something or other, which I believe is a supported Firewire chip. The USB chip has a big sticker on it. As I recall, folks used to recommend TI for the Firewire chip and NEC for the USB chip. IIRC.