RAM:
512 MB
1 GB
1.5 GB
recommended: 2 GB. reportedly 3 GB still makes a difference though.

Even though XP supports up to 4Gb, the virtual address space of processes and applications is still limited to 2 GB unless the /3GB switch is used in the Boot.ini file.
So if you go over 2GB, the performance won't really change unless you know how to pick your system.