Looking for good NAS OS

I've just built myself a NAS to act as a media server and I am wondering what OS I should put in.

I tried freeNAS but it didn't support bridging of two network cards so it was a no go. It's that the router I use is one port short and I need one more to connect my 360 to stream media to.

The web interface felt really good though, very easy to navigate in.

So my goal is to find an OS:
- with a preferably easily navigated web interface
- able to bridge two network cards to help my poor router out
- that is light weight as I only have 512MB ram and it needs to fit (and boot) on a 4GB usb stick
- that has the right protocols to be able to handle ftp transfers and streaming to 360/PCH/PS3 etc.

Also, I have an external backup drive for every internal drive installed and I would be happy if anyone have any tips on the easiest and fastest way of backup.

Thank you in advance!

Thank you for the answer but as I wrote in my original post (although maybe not so clear) it's that my router only has one port left free and that is supposed to serve both the server and a media streamer (360/PCH).