I have GeForce 6600 GT too and I've got decent play quality using medium settings with an average of 30 fps (actually fps varied from 15 to 70 in different locations).
And, well, heh, the game is very funny. <!-- s:P --><img src="{SMILIES_PATH}/icon_razz.gif" alt=":P" title="Razz" /><!-- s:P --> I cannot say there's many absolutely new and original things, but I never saw such features like gravity-shifters in serious quake-style games, only in crazy arcades like "Serious Sam". <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->
I also wonder, if that scene with player's minituarization really uses miniaturization, or it is just a fake like sending player to absolutely different location, where all the sizes are correspondingly increased?
(I haven't yet passed all the demo, made a pause on the 'Rites of Passage')
EDIT: well, tried High quality video and found out that it still runs good. (sometimes even up to 45 fps). The only scene where I got real slowdown was the "desecrated" bar in the end of the demo game, but it lasted for several seconds only.
...about turtle speeds, heh, <!-- s:wink: --><img src="{SMILIES_PATH}/icon_wink.gif" alt=":wink:" title="Wink" /><!-- s:wink: --> I remember myself playing "Return to Castle Wolfenstein" with not very fast graphics card (and unsufficient RAM), having 7-8 and sometimes 3-4 fps in large levels... but I walked it thru, <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->