All measurements taken within an hour of each other, ambient air temp about 21C (70F). The latter three machines are all in the same room.
The E2810 is my daughter's new machine and we both love it. With the Antec 3480 case, Asus micro-ATX motherboard with onboard graphics, and the E2180 + E6400's stock cooler, it is small, nearly silent, and was surprisingly cheap. Obviously with only the Intel IGP it's not a real gaming machine, but it runs all her games just fine. Overclocking seems to be limited by the memory and the Intel HSF. I debated getting something like a TR Ultra-90 for the processor but decided I'd at least try out the stock one.
The E6400 is my main gaming machine. The airflow through the Antec Sonota II case isn't as nice as the larger cases. It has a Sythe Ninja cooler which IMO isn't as nice as the Thermalright Ultra 120 Extreme. It has an SFF12F fan running full speed attached to the cooler. I ended up putting a FBA12G12M 120mm exhaust fan to really drive air through the case (2100rpm, 87cfm) which Speedfan can keep at 1200rpm most of the time and ramp it up when the CPU case temp gets to 55 (means cores are about 58). It's also got an Antec tri-speed at low for additional input as well as a little Antec motherboard fan blowing on the chipset which gets pretty hot (P5B Deluxe). Overclocking on this machine is limited by the temperature -- it could run faster if I didn't mind sustained higher temps.
The first Q6600 is a B3 I bought in August. It's a Photoshop machine running Vista x64 and we'll probably add another 4GB to it before long. It is in the oh-so-roomy Silverstone TJ-09 case which is a real joy to work with. Unfortunately the B3 stepping gets pretty hot when overclocked. I've got a Thermalright Ultra-120 Extreme, lapped both processor and heatsink. A Scythe SFF12F at full speed (1600rpm) is attached. Two SFF12E's drive fresh air through a dust filter down, and the stock Silverstone fan pulls it out. There's an additional Silverstone fan that pulls air into the front. The Seasonic SS-650HT PSU is at the bottom and pulls air from outside the case and vents it out outside. The hard drives are in their own area in the front separate from the main case and have their own SFF12D fan. Interestingly the 8600GTS graphics card ran very hot using the NVidia reference cooler -- swapping in a Zalman VF900 dropped the GPU temp almost 20 degrees both at idle and in use. While 80C temperatures seem to be acceptable for the manufacturers, they didn't make me feel good. Overclocking on this machine is limited by temperature as well as my desire for better stability since it is my spouse's machine.
The second Q6600 is a new system I just made for gaming and other use. This time I went with the Antec P182 case since it is far cheaper and seems pretty nice, though smaller than the TJ-09. Similar to the previous build I used a Thermalright Ultra-120E with full speed Scythe SFF12F fan. No lapping of processor or heatsink was done on this one. Case cooling is using the stock Antec Tri-Cool chassis fans set on low with one venting out the top and one venting out the back. I added an additional input fan to the front feeding the main chamber, and it comes with a stock third Tri-Cool fan for the lower PSU area. Obviously on this machine I have more overclock room in terms of temperature, but haven't yet felt the pressure to push voltage up and try a higher multiplier.
E2180: 60W idle, 100W load, 55.0s Prime95 2M.
E6400: 155W idle, 180W load, 45.0s Prime95 2M.
Q6600/B3: 130W idle, 210W load, 43.1s Prime95 2M.
Q6600/G0: 140W idle, 200W load, 40.1s Prime95 2M.