"Just how much greenhouse gas are you planning on pressurising that greenhouse with? There's about 6 kg of CO2 above your head and every sq.m. of the Earth - to get the same in a greenhouse you'd be looking at quite a high partial pressure. Then there's the water vapour you'd need to match what's around you. Thus a pressure cooker like situation inside that little greenhouse to match the atmosphere's IR active gases."

Well, I wasn't thinking of limiting the greenhouse gases to just CO2. For instance I believe Methane is a more effective greenhouse gas than CO2. Let's see if I can find reference.
Hmm, didn't find what I was thinking of, but "Methane is over 20 times more effective in trapping heat in the atmosphere than carbon dioxide (CO2)"
http://www.epa.gov/methane/
But there are large number of greenhouse gases many times more effective than CO2 and probably a mixture of different types could give a more effective result than one single type.

I don't know if 6 kg of CO2 per sq meter is right, but cubic meter of air is "1.222 kilograms per cubic meter":
http://www.lerc.nasa.gov/WWW/K-12////BGP/PAT/Fuel_and_Air2_ans.htm

So, in that light, 6 kg isn't too impressive, say 3 meters high, with normal air that's 3.666 kg and at 2 atm that's more 6 kg- and air is lighter than CO2.
So, a greenhouse with 10' high ceiling at with 2 atm of mostly CO2 would easily exceed the amount in the Earth's atmosphere [assuming 6 kg per sq meter is correct].
Now, you could also have about 5% being methane, and mixture of other greenhouse gases. Though probably a greenhouse with 20' high ceiling at 1 Atm of mostly CO2 would easier to do- than a structure that can withstand +14 psi.

This of course leads to my previous question:
"And would it make much difference to have a very large greenhouse or significantly higher pressure of greenhouse gases?"
By very large, I mean something in ballpark of a 100' ceiling.