One question: does DCT decimation harms encoding in low bitrate, 400 kbps or less? When I can safely use this option, without fear?

It's pretty much always safe to use it, but perhaps not advisable. What it does when off (i.e. DCT decimation is ON) is ignore DCT blocks the encoder considers small/insignificant enough not to warrant coding. Turing this option on (thus DISABLING DCT decimation) means x264 will be forced to write more DCT blocks. My advice: never uncheck "DCT decimation" unless you are using trellis AND/OR are encoding in low quantizer/high-bitrate. "it lets the encoder skip further consideration of a block under certain circumstances, sort of like the Fast P-Skip option." Logically, if you use No fast Pskip, you might want to couple it with No DCT decimation, but the size of the resulting file under CQ will increase quite a bit, and the file's quality in low-bitrate encodings will suffer, especially if trellis (which is another layer of decision making to discard unnecessary information) is disabled. Final word: If you want the extra quality in low-bitrate, turn off 'Fast Pskip" instead. "No DCT decimation" will likely do you more harm than good. edit: Read more in my guide!

New version of the guide is available. Check it out (post individual comments in a separate thread)!

The new builds here: http://amd.streamload.com/snow_xmas/Hosted/x264vfw

That's the encoder, not the guide :).

Hey. Checking out your guide I noticed that you refer to fields like Fast Pskip and DCT Decimation, while in the codec settings they actually are No Fast PSkip and No DCT Decimate. Which means they work the opposite way. :) Happy to see you alive and kicking.