Okay, if you want to overclock it, you'd have to access the BIOS and change the FSB.
If you don't like getting fried chips for lunch, try not to mess with the voltages too much. Also, get a program such as motherboard monitor (MBM, just google it) to monitor the temperature of your CPU.
What is the purpose of overclocking anyway? If you're after more performance for critical processes, its not really recommended, because there may be instability which may not present itself for some time.
If its for games, you're limited by your graphics card.
Which is why I wouldn't really recommend your overclocking it.
Having an 18Gb HDD as master is also not a very good option, unless its a WD Raptor. Looking at the other specs in your comp, you don't seem like the type who's after top end performance, so the chance that this is a raptor is pretty slim...
18gb HDDs tend to be the slower models, and also being such a small HDD, data density is low. Therefore it'll take forever for you to access data leading to generally poor computer performance (slow startup, slow to navigate, etc).
Also, you might be interested in upgrading your RAM to at least PC2700. I have personally tinkered with PC2700 and PC3200, and found that there's not much difference in them mainly because of the LAT in the 3200. Also, if you're running XP, 1gb of RAM is recommended. (the 3200 is still better, but by a slight margin).
Crossfire, he might not be interested in the Pentium D since it may need a m/b upgrade as well. I'd recommend it as well though.