If you want to be reallly stingy you can do what I did on my P-133 laptop to get it running smoothly on XP Pro (Managed to get it down to 31MB of RAM in use at idle). As well as my server which runs on a set of P3-550s.
If you are going all out.
-Disable any non-essential windows processes, and startup items
-Remove any background image you might be using
-Use classic mode for start menu, and visual themes
-Disable any basic graphics effects/transisitions/tranparencies
-Use 16-bit color or lower with the lowest resolution you are comfortable with.
-Disable any security software you can, including the windows filewall.
-Disable “cleartype” or what ever it’s called that anti-aliases text on your screen.
You “should” be able to get it around 60-70MB of usage at idle if you are using SP2. And it should be decently quick for browsing the internet and such. XP for Legacy PCs has quite a few limitations on it, but might work depending on what you want to do.