Short answer no, to all of those things.
The N900 cannot sync with iTunes, and I'd like to know what website/post told you it could. The tricky fix to that would be to download open source music player Songbird, which can import your iTunes library and can sync with the N900.
I've never heard of the adapter you're describing and anyone that tells you the FM transmitter can replace that wired connection either A) doesn't care about sound quality, or B) is afraid to knock the N900 and is just telling you what you want to hear. I'd strongly suggest keeping your iPod in your car and at home for the audio quality alone (lossless audio sounds like 96kbps mp3 over the FM transmitter, but if you have stock speakers you wouldn't really notice the difference anyway). But you get other benefits as well such as not wearing down your N900's batteries, and control over the music via the radio buttons. Touch screen while driving means you can't feel for anything and have to look away to change the track, a definite no-no. If your radio isn't stock it probably has an Aux-in on the back, which would plug in to the N900 just fine.
In conclusion, keep both. If you're just keeping the iPod around for music then you don't have to carry it with you, just keep it in the car until you want to put new music on it, then move it to the house.