With 1.2 million post, 2gb of ram is fine, but the 100k reply post, that will be the drain on the remaining ram in the system, i would go thru the DB server and disable any programs running that arent needed, i would check the ram usage at the point of load when that thread is opened to view it on the last page as well as using mytop to view the process live.
You might want to try and play with these 3 a little, but with 2gb, your pressing your luck.
join_buffer_size
read_buffer_size
sort_buffer_size
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