Unfortunately the N900 in not really supported by Nokia and definitely not pushed by them in any way, probably because it never was released as a finished product (thus, unfortunately, it and its spare parts are unavailable in many countries) and only now somewhat getting there.

Support has been relegated to the community, which is actually doing quite a good job at it (probably a better one than what we could expect from companies like Nokia, Apple or Google).

So here is my advice for flashing the N900: Download the Ubuntu Live CD and boot from it. Then follow these instuctions: http://wiki.maemo.org/Updating_the_t...firmware#Linux
Haven't tried it myself yet, but I can't see why it wouldn't work.

In any case, the N900 is and will remain not for the faint of heart. Using the device is a learning experience. For some people that's a turn on, for others it's a turn off.

Btw, you might have already voided the warranty by overclocking (which has been made tantalizingly easy), though obviously that shouldn't bar you from purchasing spare parts. I think I saw a link to a website that sells spare parts somewhere on this forums though.