Replacing your M/B is a god idea if the case is non-propriety (dell, gateway,HP). Other changes would include, power supply, memory, O/S, CPU and possibly the Hard Drive for performace.
1) Any CPU upgrade less than 1,000 Ghz will make no difference in overall speed to cost. Most boards in your class max out at 600 to 800 Mhz or less.
2) PC100 or 133 SDRAM memory is installed in your system. The new boards use DDR. (yes SDRAM boards are still out their, but why go backwords). Upgrading your memory past 256MB with win9x is not cost efficient to gain.
3) 350w power supply minimum.
Intel needs an extra 4 pin connector to board.
4) Win9x is not recommended or the boards I buy.
9x will work but not correctly or efficiently.
5) The faster the drive the faster is data transfer. You can use your existing H/D but it will be a performace hit. Also this drive is at or past its life expectancy.
6) Don't get sucked in by a snake. I have had upgraded systems come thur my shop that I would not have done. Due to age or style of system. Remember that most of your components are at or past their life expectancy.
7) I don't overclock.
If the case is proprietory then build or have a custom system built.
The cost may be more, but up-grades to system become endless.
Hope this helps and not create any confusion.