I have a Dell XPS 400 which is the sister to the 600. the 400 has an available RAM slot and a PCI slot open and a couple of other, im not sure on the 600 though since its higher ed would just call and ask dell about the open slots on the motherboard
You can upgrade the ram, CPU, HDD and graphics cards in the future. It has a 650w psu, so looking good there. I don't see anything offhand that would limit you for upgrades in the future, but of course that depends on how far in the future