Apparently, you can also tell which power supply you have by the date of manufacture code in the serial number. The first four digits are the month and year. Seems they moved from the 43W to the 65W power supply in September 2001 (0901...). The change was to accommodate the higher power draw from the RFX-32 card.
Which isn't to say that the RFX won't run in an older machine, but the power might get twitchy.