i just got mine last night.
im going to get a battery grip first. it really improves the feel of this camera and extends the batter life by 100%.
next i am looking for a canon L series lens. they have way better optics than the kit lens it comes with and can produce images comparible to something like the 1Ds mkII.
you can't get that, since the sensor on the 300/350/400d from the 1d. The algoritms on the sensor are so different on the 1D from the **0D series.
The most obvious difference is that the 1D's pics are very grey and "lifeless" straight out from the camera. They need more post-processing that the **0D pics, RAW or JPEG