Any of the rear 40g ports (there are 4 of them per switch 2x breakout, 2x stack) can be used for stacking. However the 2x breakout ports cannot be used for 40g, only the 40g only ones. (Fohdeesha just answered my question on this yesterday)
So what I am going to do with mine is stack across the breakout ports, and use the 4 total ports across my 2x 6610's for 40g.
The max ports per 6610 you can get straight 40g is 2x per switch (if you disable stacking and/or use stacking on the breakout ports).
As for POE - I hadn't seen much difference in price on the 6610's POE vs. non-POE so you might as well get a POE one. By default POE is disabled and you can enable it per port when you need it. I believe the switches are built with the same number fans/power supplies.
Make sure you get them with 2x power supplies and 2x fans as they are expensive/hard to come by (especially the fans). Also keep in mind the exhaust direction - the E models are rear exhaust direction
If you are looking for something not as loud the 6450 is the way to go but you have less ports/no 40g no dual power supplies. I would not get the 6610s if they are in the same room as you are.
So what I am going to do with mine is stack across the breakout ports, and use the 4 total ports across my 2x 6610's for 40g.
The max ports per 6610 you can get straight 40g is 2x per switch (if you disable stacking and/or use stacking on the breakout ports).
As for POE - I hadn't seen much difference in price on the 6610's POE vs. non-POE so you might as well get a POE one. By default POE is disabled and you can enable it per port when you need it. I believe the switches are built with the same number fans/power supplies.
Make sure you get them with 2x power supplies and 2x fans as they are expensive/hard to come by (especially the fans). Also keep in mind the exhaust direction - the E models are rear exhaust direction
If you are looking for something not as loud the 6450 is the way to go but you have less ports/no 40g no dual power supplies. I would not get the 6610s if they are in the same room as you are.