Doug, do a search. Someone already did the homework on the fastest processor you can plug into the board. For the most part, if your use is only HTPC and not desktop or game machine, then you are fine as is. These machines are overpowered for thier intended use. I've seen 1GB machines perform the tasks just as well.
The optical drive is a no brainer. Anything is better than the OEM drive. It's IDE, plug and play swap. Fans are about the same, just make sure you pay attentions to the dB's. The biggest bang for buck is the vid card. The stock cards run hot, and this makes the fans tun overtime. Upgrading the card will payoff bigtime. But wait till yours fails -- it will. Memory is the next biggie, it adds capacity.
Hp DEC Z558
XP MCE
Two yrs under the belt, only sent back twice!
New vid card, new DVD drive.