Just wondering, of all the cards listed by Sony for the NX5, which is the fastest?

Memory Stick PRO Duo
Memory Stick PRO Duo (High Speed)
Memory Stick PRO Duo (Mark2)
Memory Stick PRO-HG Duo
Memory Stick PRO-HG Duo HX

How do they compare with SDHC class 6 and class 10 cards?

You are basically paying far more than you need with these cards, SDHC cards are far less and just as good, depending on the brand...I use two 32Gig class 6 Transcend SDHC cards and have never had any problems whatsoever with my HD footage with my AX2000 and my Canon HG21......and transferring my files to my PC is very fast as well..the big plus is I payed less than $100 for each......

Okie dokie. Thanks David.

And because I am a paranoid, I use more 16 gig cards rather and fewer 32s. Price is about the same and I only loose half as much stuff when one of the cards goes missing or craps out.

For the same reason I use only 8 and 16 GB cards.

In my opinion, using more smaller size cards only increases the chance of one going missing or corrupting while mounting/unmounting.

I currently have one 32GB card and one 16GB (came with the camera). I'm saving for the 128GB flash unit which will then make my SDHCs just a backup recording option now that the dual recording bug has been fixed.

I've never heard of corrupted or missing files during mounting/unmounting. Instead you can read on Panasonic HMC150 thread about the cards malfunctions. I must say NX5/AX2000 has a much better reputation (despite the simultaneous recording bug that had been fixed eventually) and nobody reported any problems with the memory cards.