I have spoke to Matrox and it appears that the upgrade you buy unlocks the capability of the parhelia chip to give you triple head. Another thing to note that is the triple head capability only gives you analog outputs. The upgrade effectively give you the same capability as the P750 but without the fan

The official upgrade can only be purchased from Matrox online, don't know the reason why, but to purchase it you need to enter the serial number of the P650, you pay about 78GBP incl. packaging, and then you get the cables and the software to unlock the card.
This was my impression from the info i have seen. HTH.