The early 335's (2006/early 07) had a common fuel pump problem:
I also agree with Minichamps, if you got your 323 brand new (or if it is reliable and trouble-free) then why trade it in for a used one?
I upgraded from a E90 320i to the 323 - initially it was a huge improvement over the 4-cylinder. Now seven months later, I feel like I can do with another 20-30 more horsepower!! Yes the 335 is very fast, but maybe after a year or so after you have upgraded, you might be asking a similar question "335, M3 or 911"... You can never have enough power!!
In addition, my C230 Touring (2.5 V6, 204hp) feels a bit more responsive and has a bit more torque than the 323, even though the 323 accelerates quicker to 100km/h on paper. Might be the 7-speed transmission...
If you can hang onto it for another 2.5 years or so, the next generation 3 series should be around in 2012!!
BMW E92 323i, MY2009, Space Grey, 18"s;
BMW E70 X5 xDrive30i, MY2009, Space Grey, 18"s;
MB W204 C230 Touring, MY2009, Calcite White, 17"s