OK, seriously then

* PIII usable
* work with 128 MB of ram
- these specs should pose no problem; I think the original knoppix will run fine with that, choose a light window manager and it should run fine....

* boot time less then a minute (eben if no fuky hw will not be automaggically found), at leats dhcp+mouse+drives+video needs to be found.
- Even if you cut down on hw detection et al, I don't think this is a goal you are going to achieve, CD loading is slow... maybe a (partial) network install; ie. pull all the big programs from a central server? or boot from a different medium (HD?)... even then I think 1 minute is unlikely on the specs you stated, even my P4 with SCSI takes several minutes to boot

* save home dir in usb disk
- I haven't tried it but I think knoppix supports this by default

* graphical boot splash (no text at all if possible, see pclinuxos for example)
- should be no problem, the image patching howto's are all over the forum, and in the remastering-howto's

* 2.6 based is better, since most of my box is not supported on 24 (usb + sound
- so sound + usb of the default knoppix does not work properly on your hardware? that I haven't seen a lot, but there are plenty of 2.6 guides on this forum

* 650MB iso
easy, once you start remastering your knoppix simply remove all non-essential apps and you should have loads of space

* installer
- ??? install what to what?

* single theme with gtk/qt/mozilla/xmms/mplayer/whatever apps
- sure, I want that as well. It's easy; set up the knoppix user just the way you like it and then copy those config files to the /etc/skel

* nvidia+ati drivers
- Sorry, weren't you building for a programming box? While this is possible (but you probably will need to do some compiling, sorry!) I do not see the need for 3D on a workstation, as the 2D drivers work fine

* flash, acroread (propiraty, "free"/non_gpl software is cool by me)
- available from the debian repositories

* c++ ide (kdevelop) + kde includes
- I think most of this is installed by default, kdevelop certainly is

* if i have place, java + ide
- big, bulgy, but if you do some radical package cutting and remove all cruft it could fit...

I think you need to refine your goals a bit, and just start playing.
Personally I would recommend following one of the remastering HOWTO's to build a chrooted knoppix master building environment, clone that drive (so you have a backup baseline config to fall back on when (not if) you screw up your master, this will save you time) and once chrooted; apt-get update to get the latest packages and then install synaptic to have a look around... there is so much software in the apt sources I think you will meet 90% of your goals without compiling a thing.