Just to let you know guys that we have actually done it (with the the help of OPC Reading and the knowledge from other posts on this forum) and all for around a £1000.
We found a 996 hardtop on E Bay the same colour as our car that came off a 2002 996 C4, paid £400 which we were very happy about as a good paint job could of cost around that alone if we had bought one in another colour to the car.
As you guys will know from reading the post above these 996 hardtops (whilst having the same part number as a 997 hardtop) have the older male fittings fitted, so these need to be changed to the new threaded type.
As our 997 was originally ordered new from the factory without a hardtop it did not have any female mountings fitted to the car, so we also needed to purchase two of these which we got Porsche to fit for us (1/2hour labour).
Once we had these mountings fitted we then fitted our Hardtop and took it back to OPC Reading who very kindly checked it and made a few slight adjustments so it sat safely and snugly on the car, (watch you don't fit it to tight or it might scratch your paintwork, to lose and it will rattle and you will get wind noise).... and yes even the heated window works.
Part numbers required were the two newer catches for the hardtop ;
P996.563.403.04 061763/catch c-pilla @ £156.87 each
P996.563.404.05 061783/catch c-pilla @ £156.87 each
Then the two female mountings that fit to the car ;
P997.561.653.00 mounting @ £91.81 each
P997.561.654.00 mounting @ £91.81 each
and finally 8 bolts ;
P900.378.173.09 to fix everything in @ .30p each.
All prices plus VAT as of 22/10/2010
This post is just for info so please check yourselves with Porsche ref part numbers and costs, however be warned, a few OPC might tell you it cannot be done, but yes it can if you are armed with the knowledge......