I was in the same boat as you are. I have a 4830 also and did replace the HD with a 20 gB HD. However, the bios won't recognize a drive bigger than 8 gB so you have to install the MaxBlast software or the Western Digital equivalent. Still, it did speed things up to get a 7200 rpm hd in there. The Quantum Bigfoot drive is a real dog. If you insist on upgrading the Compaq, then a new HD and some memory are your best bets. Put as much ram as you can afford in there (at least 128 MB) and a 7200 rpm drive and it will be faster, but still old technology. You can't fit a new mobo in the Compaq case since they use a proprietary design. You will have to start from scratch.
But as a previous poster said, the Compaq 4830 is pretty old and you would be better off to just replace the whole thing. I built a new system using my old monitor, DVD drive, keyboard mouse, printer, etc for under $500. I got a nice case with 400 W ps, Soltek SL75DRV mobo, 512 DDR Ram, 2x20 MB HDs, ATI Radeon 8500, Athlon XP1700+ processor. It is a much better machine and I still have the Compaq.
I will use the Compaq to run software for my daughter until she is old enough to notice how slow it is.