What size power supply for i7920 with both Quadro FX 1800 and GTX470?

I would get the Corsair 750 for that build. There is a PSU calculator here, if you want to confirm your choice (and add the other components you plan to get).

What's the point in pairing a quadro with a geforce (actually am not sure if it can be done also as both use different drivers)..!! More so when both are from different architectures.. The quadro cards also have CUDA cores BTW..

the whole setup would be iffy-i would just use either the geforce or the quadro not both.

Apparently this is a pretty common setup to use with Octane...
Basically you are using the quadro for all of the visualization work and setting up the gtx470 only to farm the GPU rendering to. There is a bunch of info over on their forum if your interested. thanks for the info though!

Spent some time studying the Octane architecture.. It looks like quadro + geforce pairing is possible afterall.. But still, am not sure that the current Fermi is worth when, only used for CUDA cores, at those power draws and heat dissipations.. Might as well wait for the Fermi based quadro's to come out.. Or, if you are really focussed on the pairing, then i'd suggest you to give the nod to the quadro for now and only get the GTX 470/480.. Then may be you can pair it up (if required) with the relevant quadro card when they get available.. By that time, you might get an idea also as to whether you require the quadro or not..

Thanks for taking the time to do some research. I would love to just get the 470 and be done with it until something better comes out but I am a little worried about it's performance in some of the software that I use. Mainly acad, revit, 3ds max design and sketchup. Any thoughts on the pairing?

Nvidia has specialized software drivers used with their quadro line of cards for apps like 3DS Max.. Other than that, the only areas where the quadro excels compared to the Geforce are -

1. Better color accuracy (you require an equally able monitor to experience this)

2. Lower power usage and heat dissipation

3. More stable drivers and seamless support..

If You've worked with a quadro card before and know how to utilize it fully, then sure go ahead and get one.. Performance wise, there is not going to be that many differences IMO.. If you are really interested in pairing then do like i suggested before.. Get either of the GTX or quadro version for now and check if it gives satisfactory results for your work.. You can add later as per your needs obviously..