What's the "sufficient" quality for a portable player of 20gb.

It's q2 ok? or is too low?

It all depends on your needs, listening environment and headphones you will be using. I don't use ogg vorbis but I use lame on v2 setting for my 30gb video ipod (and I think its too high for the portable player, I could go lower but its easier that way, I can have the same mp3s for PC usage).

I agree with mat128's sentiment and think most people here at HA would also agree with it i.e. it depends on your ears, where & how you'll be listening, how much stuff you want to put on the player etc etc; basically finding the right balance for you between size and quality.
The best advice I can give in addition to that is to experiment a bit and see what works for you; using the sped-up "optimised" Lancer encoders might save some time (and Lancer + oggdrop is fast and easy IMHO ).
Just by-the-by, I tend to use q2 on my player and am very happy with it (but am looking forward to the next aotuv release too).
Hope this helps.
rc

I've had great results with 100k to 130k AAC and MP3. I suspect that Vorbis would be comparable.

Think Q5 is overkill for a Rockboxed iRiver H140 using (cheapy) Sony MDR-A35G headphones?

I would say yes but your mileage may very. Try down to Q1 in your intended listening environment.

Usually, the sweet spot for portable is around the lowest quality setting that is still transparent to you on your computer (or a step lower).

It all depends on you ear... and the song... and also the playback environment.

For Trancemixes heard in buses, -q -0.7 works for me. If not in buses, then -q -0.5.

For R&B and altrock tracks in various settings, -q 0 works for me.

For easy-listening vocal tracks, at least -q 1 for me.

My Rockboxed iPod mini 2G (the one with a 4 GB hard drive) plays -q 6 without problems, so that's what I usually go for. The quality seems transparent and the amount of music is more what the battery is capable of playing anyway. So I guess my default habit optimizes the need not to re-encode again.

20 Gb sounds like that if you don't mind hearing artifacts, the aoTuV pre-beta 5 could easily mean something like 13.000 songs in your pocket.

Well, go as low as your ears will let you.

Personally I would use (and do use) -q 0 --advanced-encode-option lowpass_frequency=99... but thats just me.