Well I believe in theory 10-bit resolution should give 1024 levels (2 to the power of 10) and 12-bits should give 4096 levels... But I'd be very surprised if the N900 screen really does support that many levels.
Remember that things could be much better if we got this bug fixed: https://bugs.maemo.org/show_bug.cgi?id=8435 I don't know if any of you have tried mypaint on diablo (n8xx) - it is pressure sensibility is a dream compared to fremantles crippled version... I believe it is a software bug not related to hardware - Mer on n8xx also had this pressure bug. Vote for the bug now or it will be forgotten as Nokia moves towards capacitive screens!