Installing the G.729 codec is straightforward, you simply add the required pieces of software to the machine running asterisk. As I understand things the codec is restricted to that machine only after registration. So you download, Run the registration tools. Done.
This is the way its done with digium purchased codecs.
Now as far as G.729 being 8kb/s, its true. The payload is 8kb/s but there is ip overhead to consider, and from what ive gathered with G.729 the header information can outweigh the payload. I find that funny.
GSM - 13 Kbps (full rate), 20ms frame size
iLBC - 15Kbps,20ms frame size: 13.3 Kbps, 30ms frame size
G.711 - 64 Kbps,
G.722 - 48/56/64 Kbps
G.729 - 8 Kbps, 10ms frame size
Speex - 2.15 to 44.2 Kbps
ITU G.726 - 16/24/32/40 Kbps
I took the codec info from http://www.voip-info.org/wiki/view/Codecs