You need 2 files:
CamStudio 2.7_r316 http://sourceforge.net/projects/camstudio/files/latest/download
x264vfw Codec http://sourceforge.net/projects/x264vfw/files/x264vfw/37_2200bm_33968/x264vfw_37_2200bm_33968.exe/download
Download and install.
The followings settings worked well for me (Windows 7, DualCore processor):
Video: x264vfw, Q100, key200, frame25, 40
x264vfw config: ultrafast, CRF23, Loglevel=none
Audio: 22kHz, mono, 16bit, PCM(change has no effects!), Interleave1000ms, no MCI!
Audio and Video Sync.: 500ms delay (otherwise time costly videocompression leads to timedelayed video!)
Because I get always PCM audio regardless which audio codec I choose, the only way to get smallest filesizes is to compress audio afterwards.
For later audio compression e.g. mp3 with Virtualdub make sure to use CBR (not ABR), otherwise audio is not in sync!
CBR is only available for samplingrates 32kHz or 44 kHz, so you have to convert from 22kHz to 44kHz afterwards (to 32kHz gives bad quality!) or choose 44kHz in Camstudio settings for recording beforehand (bigger filesize-> 2GB limit!)
A test recording of a 91 min webinar (with video) format 862x644 resulted in a size of 544 MB without audio compression, i.e. 5 hours recording without reaching the 2 GB limit should be possible. audio and video parts in the file had roundabout the same sizes.
I plan to make a test with 720p standard resolution 1280x720. I will post here the attained filesize.
I failed in trying to compare codecs x264 and Xvid regarding quality vs. filesize by now, because after choosing Xvid video codec (Jawor´s Xvid) camstudio doesn´t start recording but stops with error message "Wave in error: Das angegebene Gerätehandle ist ungültig. in Stop()".
Edit: Please make sure that both, region width and height, have even numbers! If not you´ll get errors while starting to record. To make sure this doesn´t happen use Region->Fixed Region instead of Window. It´s annoying to be forced to change windows size by one pixel if you need to record NOW.