For a 32 bit OS the base requirements are min 1GHz Pentium 4 CPU and 512MB RAM + additional 64-96 MB RAM and 100 MHz CPU speed per user.
So as a 32 bit OS is limited to max 4GB RAM it could do approx 30-40 users with a Core 2 quad and 4GB RAM for average office applications.
With a 64 bit OS you could use as much RAM and number of CPUs as the Windows OS version supports.
e.g. XP x 64 supports max 128GB RAM and 2 physical CPU sockets (e.g. 2 core 2 quad or 2 quad xeon CPUs which gives you a total of 8 CPU cores to use)
So for 150 RDP users we would recommend XP x 64 with dual quad core Xeon CPUs and min 16GB RAM (better would be 32GB RAM)hardware, minmal, requirements, settings, specification, system, XP/VS Server