So Im thinking of selling my N900, but when i send it of to whoever buys it, i want to be sure that there will be no way for that person to recover any of my personal information. I have yet to try re-flashing the device, simply because I'm still using it as my main phone, but I want to know if you are still able to recover files from it using file recovery software. Is there anything that I can do to make sure the whoever gets my phone won't be able to recover any information?

After flashing, how about install rootsh , ssh and use SCP to transfer small junk data files (10MB each) to fill up the space.

When the disk is full, wipe out again. Then even if they are able to recover using recovery software, only junk files could be recovered.