Changing your motherboard, while its a major update to your system, really isn't that difficult - just kind of time consuming. Any board you get will come with detailed instructions on where everything gets plugged into. The nice thing about motherboards is that there is a standard size (ATX) so you don't have to replace your case. Its a matter of disconnecting everything attached to your current one, pulling it out, putting the new one in, and reconnecting everything. The other nice thing is that cpu/mothreboard/memory/video tend to progress at the same rate technology wise - eg. newer processor requires a newer motherboard and newer memory, etc... but this works in your favor because finding a motherboard compatible with your processor means the chance of it working well with your other components is very high (so you don't have to upgrade anything else, just the MB).
Most manufactured systems (dell, gateway, hp, etc...) use the tamper proof BIOS as a safety and stability measure. They get far fewer tech support calls and warranty replacement issues when they know that people can't push their machines farther than they were designed to go. Overclocking results can be pretty processor specific - a celeron 2.5 may be able to push to 3.0g while a 2.6 may only go up to a 2.75 (these are made up but the idea is solid).
www.tomshardware.com is a great resource for speed comparisons and OC info. Successfully overclocking your current cpu will definitely require a new motherboard, and you may see results in the range of a 15%-20% gain. I'd recommend taking a look at a new barebone system
tigerdirect were for a pretty modest price you'll get a new computer that out of the box is 100%+ faster, with potential to be OC'd even further. Its just a recommendation though, money is always an issue for everyone. Again, a new MB for you should be pretty inexpensive, and the boost may keep you happy for another year or so which could really make it worth it. If you're trying to OC for a specific reason like gameplay isn't very good - that's much more dependant on your video card, so perhaps an upgrade in that department would be more worth your while. Just a thought.