i to have had the same problem as everyone, uploading to xbox really slow, starting at around 350k/s, then going down to 0k/s. I to have tried everything, different network cards, different ftp's, the works. I did have it working fine in the past.
I got fed up and formatted my computer. I re-installed WinXP Pro, MoBo drivers, SP1 and SP2. Then i installed flashfxp (which previously used to crash). I disabled passive mode, and tried to transfer 9meg/s!!!!!!!!! Seems like the format did the trick.
I then continued installing software, Office, email, browsers etc. Then i installed Zone Alarm. I then repeated my Flashfxp experiment. Back to slow speeds and crashing flashfxp. Disabled (closed) zonealarm, still no better. Uninstalled zonealarm. Restarted computer. Tried FTP again, back to 9meg/s!!!
So even with zone alarm turned off it was still interfering, it needed to be uninstalled.
I hope this helps some of you other guys. Post your results.
For the record windows firewall is currently on, and ftp is working fine with it, i know its not the best firewall, but it'll do till i test some more.