A PCI ATA or SATA card and a modern hard drive will give you vastly better disk performance than what is available from the built-in SCSI and older hard drives. But don't make the mistake of adding an ATA interface and then installing a very old IDE drive. The platter/media speed on those really old drives couldn't deliver much in the MB/s department. Get a drive which was made no later than 2003 (that's a rule of thumb and will have exceptions).

If you can find one, you might consider a SIIG USB2/Firewire/Gig-enet card. It very economically gets all three functions on one card. Hens teeth now days.

In my mind, the perfect combination for the three PCI slot machines is an Acard 6880M (or 6280M), the aforementioned SIIG card, and some video card, probably a Radeon 7000 or 9200.

There was/is also the 1000MHz PPC750FX based G3 card from PowerLogix for the PCI Macs. I don't know if those are still available. You might be able to fake it with a ZIF Carrier Card and the 1GHz G3 ZIF, which I think is still available.

But really, all that upgrading is only good for fun, and if you can afford it. In terms of simple price/performance, the used G4 machines from Apple are a better value, rather than trying to upgrade a 7600 -- unless you need to run something pre-OS 9.