We had to purchase i5 iMac which was released today because we have the trade show next week and we couldn't get the i7 iMac for demonstration.
We upgraded the main memory to 8GB, makes it just around $2,000.
I connected it to Small Tree GraniteSTOR via 1Gb ethernet (with their specially configured switch) and tried the m size proxy editing on FCP7 and it worked well. Felt really comfortable editing.
Since Matrox MXO does not support Snow Leopard officially, the mastering mode seem to have glitches, but the digital cinema desktop or the presentation mode work fine. If you are okay to work within 64 to 940 video range then this will be fine.
We find that all LED backlighted LCD display is shifted towards magenta on high range and I see this is true on this iMac but that can be adjusted with calibration I suppose.
We are going to see if MOTU V4HD works well with i5 and i7 iMac in this environment, too.