First off you have a computer that cannot easily be overclocked, because it's motherboard lacks a bios that allows for overclocking, namely the ability to change FSB and voltage. There used to be a program out there called softfsb, but it has been discontinued. That might be your only chance to do anything with your current system...
Secondly, you could buy a bx motherboard and likely a new case with powersupply and swap over your components, however the gains will likely be very limited. Most PIII 450s make it to 560, some go to 600 and few go beyond. This was hot 2 years ago, but nowadays, where we are approaching 1.5GHz systems it's not a good suggestion anymore, and really not worth much time and money.
If you really want to get into this hobby hang around here for a few days and follow some of the discussions. If you want to pursue a new system, then decide which board and which processor and what case to get. As a minimum I would expect not much less than 1GHz overclocked (eg a 700MHz PIII or TBird at 933 or above) . A good value are curently the AMD AXIA 1GHz processors which do 1GHz to begin with, but are likely to reach the 1.4GHz range if cooled appropriately.
You can likely swap all the other components over, but you will need a new case and a new powersupply 95W is not going to cut it in any overclocked system.