You can install some updates, but not all. Going to 10.5.2 is easy, past that things can get a little trickier but you can still use Apple's updates with workarounds, or hacked updates without them.

What motherboard did you get? The P35 Gigabyte boards have good compatibility and even a nice GUI app to install drivers. Onboard sound, network and ports (even PS2) all work, I think the parallel and serial ports do too but haven't used them. Very nice to have a Mac with 8 USB, 6 SATA, 2 sound outputs and a real mic input port.