If you wanted to have a fully updated system then you should install everything what is included in the HFSLIP archive but in reality you can just do this to update system core components. Install:
- USP5.1 from here (w2ksp51.7z)
- KB2797052 (I know that there's a newer IE Cumulative Update available but I haven't been able to add it to the Download Archive yet)
- Update Rollup 2
- the latest weekly release of UURollup-v11 (available here).
After that you can play around with XP drivers (but need to modify their INF files in most cases) and also try to enable PAE:
Intel Physical Addressing Extensions (PAE) in Windows 2000