Well, i got 2 reasons for transcoding my mp3s to ogg. First - it seems oggs are smaller, second - i like the idea of having non-patent audio format on my pc But, i also know it's not a great idea to do lossy-to-lossy format transcoding. Some guy here on forums wrote he could achieve transparency doing this on q6. I'm curious if it is true and i'll be very happy if somebody could confirm this or give me any tips on how to do that.

Waste of time unless the mp3 is very high bitrate (260~320k) and you want OGG files for portable use. By all means try and see what is acceptable to you.

Transparency can be achieved with q6 when encoding from a lossless source, but artefacts may become audible when transcoding from already lossy files.
Plus at q6, the Ogg Vorbis files probably won't become much smaller than your MP3s already are, so reason 1 vanishes.

Ogg Vorbis excels MP3 in the low-bitrate region, so for a real saving of space, you could reencode your CDs to Vorbis using lower q values like q3 (~112 kbit/sec) or q4 (~128 kbit/sec). Perform ABX tests to find out what is transparent for you. There are people here who live happily with q1 or even less.

Thanks for your responses and advises. I think that making ogg files from audio cds will be the best way to solve this.