I went through the same exact thing, buy a new or slightly used E46 M3, or hold off for the 335i. I went back and forth for a few months, researching as much as a I could and debating the pluses and minuses. Ultimately after a few months, I finally decided in March to wait another month or two for the 335i official pics to come out. When they came out in late April along with the specs, I knew I had made the right choice; it definitely was a hard decision, but the right one.
I went for the 335i, newer design, newer technology, newly designed engine, more torque, twin turbo so easily to mod, brand new 4 year warranty, new options like comfort access and integrated ipod. I like that it's 4" longer too. Thats not a lot, but enough that this will help out a little with leg room and space in the backseat. Plus, a lot of people already view the E46 M3 as the "old M3", its been out since 2001 and was designed in the late 90's. The design and technology and engine are all at least 8 years old. I think it's still a gorgeous car and it was all I could do not to pull the trigger on a 2005 ZCP M3 in Interlagos Blue.... but still, it IS a dated car by now, and there have been VERY little revisions to it since released in 2001. Imagine it now if you only keep your car 3 more years.
Topping it off, the 0-1000 times look like this (both manual):
0-1000m for 2006 E46 M3: 24.2 seconds
0-1000m for 2007 E92 335i: 24.5 seconds
Most reports are saying the real world times for the 335i in daily, usable situations will equal or just barely beat the E46 M3 (such as 30-50, 50-70 acceleration, etc.)
Now imagine buying an ECU chip.
All that, for about 15k less than the E46 M3 (comparing fully loaded vs fully loaded).
I would never tell someone that buying the E46 M3 is a bad decision at all. Its a great decision. For me though, after weighing everything, the 335i twin turbo was just an even BETTER decision.
Aija, you should add "warranty/no expensive repairs" to the + section of the 335i, and a - to the M3 side for the drop in value due to not only the 335i coming out, but also the E92 M3 due out in probably 12-15 months.
Now of course, when the E92 M3 comes out, this will be a whole new ballgame.
If you decide to go with the E46 M3 anyway, make sure you use any and all arguments you can find to get a solid 10% off if not more. They should give you a decent deal with the E46 M3 design being antiquated and this model design of the M3 being phased out.