last time i installed XP on 1GB - 2GB system's and was around November last year,
it was about 20 services being disabled in XP 32-bit version,
only a few services (system restore, indexing and uPnP device host related) have been disabled in XP x64 edition.
when the system was 512MB i used to install Windows XP with more than 30 services disabled, that was more than 4 years ago.
All Services's Descriptions are in the Services MMC Snap-in.
the information in the Dependencies tab panel on each service's can also help you decide which service should also be disabled when you want to disable or change its startup type of any services.