There are a number of PCI options available. Internal drives can be hooked up to some cards. USB outlets can be added. I'm adding two PCI cards to my StarMax. One is an ethernet card from Farallon so that I can go to LAN parties, and the other card is a 3-D graphics acceleration card.
RAM is often the first upgrade for any computer.
My StarMax came with 16 megs, and it wasn't long before I added another 16. But even though I could use more, I'm putting that purchase off until after I've installed these PCI cards. Quake 3 Arena and the recently released Quake 3 Test demand some sort of acceleration. There's really no downside to installing a 3-D acceleration card. The card's going to take much of the work involved in rendering the screen off of your computer's processor.