TL: DR? Read bold parts only.

I have been using XP for quite a long time and learned a lot about it.
I mean a lot.

First I was a total newb and even got viruses and things like that. I reinstalled XP a lot of times.
I started first from simple things like MSCONFIG and Visual Settings.
I then went on to Services and learned alot about them.
Then applied a mass amounts of registry tweaks.
I had an old AMD3000+, 1GB 333Mhz DDR, and a 6600 and wanted to play Bioshock at good FPS.
So I started tweaking further...

Turns out running XP below 120MB was a challenge followed by running XP below 70MB. But I managed to get it to run without UI, at like 48MB, and 52 with UI and 20MB without drivers. Then I had to reinstall for that time I removed to much of Windows itself for anything else then SP gaming. :\

Anyway, I am trying to optimize my Windows for performance again, without removing most features - just disabling them. I already applied many tweaks, regedits, NTFS tweaks, and stuff.

Any advanced tweaks you could recommend me? Like things that are not at all common but really awesome?
I am currently thinking of deleting multi-user environment in XP to speed up boot by making some major changed in the registry. And also for 32-bit environment I am thinking about reshacking the dlls and exes to modify and remove useless items and pictures as well as add some items. This will be a major undertaking, anyone have any tips aside from backup?