I agree with Steve on his points, most of those tweaks are for compatability and not relevent for the most part. Two very useful sites to visit to reduce the memory used by Windows XP include http://www.blackviper.com and http://nuhi.msfn.org/ I have used both on the WinXP PC I am using to type this, and my memory use at startup is 48mb, 3mb used for task manager, so net 45mb of ram. Much better than the standard amount with all the services and tools installed by default.
Using the tool on the second site can leave you with a neutered OS though, my first pass included total removal of IE, and I couldn't open MMC to add users, and couldn't open key control panel apps and utilities that rely on integrated IE components. Leaving the IE core components solved my problems, and it cut the installed size of the OS to 350mb+pagefile. I think the default is over 700, so that's handy for a smaller HDD. My first version came in at 250mb, but for all it could do, Win95 would have worked just as well.
The main benefit I've seen is the speed of loading once you're to the login point. For a while I had it auto login as admin, and once that started I was at a working desktop in about 2 seconds. I know there was still a process running in the background, but it did shave a couple seconds off what I'm used to seeing.
Word of warning, I haven't installed FrodoPlayer or other unique apps yet, so I can't say if that works. But FireFox, OpenOffice, and a bunch of plugins/small utilities have worked fine so far.