Personally, I love WinXP...but as this forum can attest, your mileage may vary.
However, I would do a clean XP install rather than upgrade it. Most operating systems don't upgrade well, because you are mixing old code with the new. For a WinME to WinXP...this is a lot of new code.
Also, there are a lot of programs that have been updated for better compatability under XP. If you upgrade from WinME these programs will still be still optimized for WinME unless you reinstall them. Windows XP may have you do this anyway.
Where to find it to download???...many people have been finding on p2p networks. But sometimes these images are incomplete or messed up. I would wait for a listmember to post a URL that they found to work.
As far as tweaking (and you will want to tweak it!)...you are already in the right place.
Just use the search button.