CO2 is only cold when it is expanding (boiling off). A waterblock filled with CO2 would be no better then a waterblock filled with water and a dead pump (it would fry the CPU). You could get a compresser in which case you would have invented a homemade air conditioner. I'd recommend a regular air conditioner or better yet a watercooled computer
The Maze2 is one hell of a block. DangerDen makes both 3/8 and 1/2 inch versions. IIRC the blackice is only 3/8 inch. So if you want a 1/2 inch system you would need a Big Mama or a standard heatercore (generally have 5/8-3/4 inch fittings). Oh yeah, the BigMama by stock is also 3/8, but for $5 extra they will mod it to 1/2.
Waterblocks need to have as much waterflow as close as possible to the CPU core. The Maze2 does a very good job of this. A standrad HS would probably pass most of the water through the fins and not the base where its need most.
There was a long thread on
www.hardforum.com a few months back that discussed 1/2 vs. 3/8 in great deal. I went with 1/2 in the setup I ordered today.