Sorry for the long post, ahead of time:
According to the other thread you started in Tech Support, you have a gateway system. Finding any sort of overclocking mobo for this particular system would prove fruitless if you intend to keep most of the parts and re-use them. You most likely cannot use the same power supply or case with any mobo not put into the system by gateway themselves. Basically, bare minimum you'd be looking at getting a new mobo, case and power supply. Doing so, however, won't give you any kind of noticable performance gains in any respect. In the end, you'd still have a P3 600MHz, on a board that'll let you get maybe another 50 MHz out of it, and PC100 RAM. Not to mention the old vid card. It would still be a really darn slow computer, albeit a slightly-faster-than-it-used-to-be one.
Your best bet would be to start from the ground up. However, a few fundamentals must be answered before we can give you any sort of buying advice. What is it you are planning to use this system for? If you are looking to do some gaming on this thing over the next couple of years, I strongly recommend against trying to upgrade this system. You'd just be throwing money away. If you're just looking for a guinie pig to play around with, and that system is just laying around anyways, then save your money and start taking things apart and putting them back together. At least this way, you're not out any more money than you need to be. But if you really, really want a new fast system to do super crazy things with, then I'm sure everyone here will be more than happy to walk you through building it and setting it up.
As far as how difficult it is to put together and set up a new computer, I'd say anyone who has enough common sense to figure out how things work is capable of getting everything together, albeit with varying levels of headaches (or in some extreme cases, migraines). I myself first started tinkering with computers when I was 14, and have since built several systems for friends and family. I'm not a professional builder by any means, but I do enjoy doing it. Basically, think of it like this: Most of the stuff is pretty rudimentary, and most of the components cannot, without extreme force, be inserted incorrectly or assembled incorrectly. But there are things that can be done wrong. As long as you pay attention to what you're doing, take your time, and read and UNDERSTAND the manuals, you'll be fine. Getting the hardware together isn't much more difficult than adding an expansion card or memory, however setting it all up in software and BIOS can get tricky. But all that stuff is in the manual and on the CD's that come with the new hardware (ideally). If you are unsure about anything, post here first. It may end up taking you a bit longer than you'd like, but everyone has to start somewhere. If you have any fursther questions, or if you can help clarify what it is you want this system to do for you, then just let us know.
And for all you nostalgic types, the first system I fooled around with was a 386 SX 15MHz, or something around that, although the oldest comp I ever messed with was a 286 sometime after that. I'm afraid to say, she didn't make it through surgery.