My .02 cents.
On my younger siblings computers, I recently installed WinME on both fo them since they don't really deal with computers all that much. And I must say I am impressed. It doesn't seem that much different to Win98se in terms of speed, but the multimedia capability and system restore are great features. When I was home, I hooked up my Kodak DC265 digital camera, and didn't even need to install drivers. The OS recognized it, and transferring and deleting was much faster through the ME OS than through the original Kodak drivers on Win98se. The system restore has also kicked in once or twice saving me a lot of trouble.
Of course on my newest rig, not even a week old, Win2K now refuses to boot, and I have to wait three more days before my friends get back so I can make an ERD disk on their Win2k boxes. I wish Win2K had as easy of a system restore feature like WinME.
To answer your question, I don't think it would be wise installing WinME on that 200mhz computer. I once tried installing 98se on a 90mhz pentium, and it kept hanging during the install. Had to revert back to win95. Of course, 90mhz and 200mhz is a big difference. If you are having problems with Win98se, which it doesn't seem like you are, then go for the upgrade. If not, stick with what you have.
By the way, I'm pro-Microsoft. Even though Win2K crashed on X-mas day... even though I still can't use my Win2K box as of now... and even though they've given me so many headaches in the past... kind of stupid that way.