1. Compatible with Ubuntu. Well, it is rather hard to design USB storage that would work without driver installation in XP, but not in Linux. And being Windows-usable without installation of a load of spyware is an expectable drive property, so you can check it when you have nearly decided everything.
2,4,5. USB-powered and being lightweight means 2.5" disk. 500GB means 3.5".
3.Fast means high-RPM 3.5"... Quiet means 2.5". And reliability is hard to tell for new models.. If I would want maximum reliability, I'd buy separately an USB enclosure and a 500GB HDD which is being phased out but still is not synonym to failure in Internet. In case of IDE, better test that USB enclosure supports anything above 127 binary GB.
6,7. I feel like spending some effort and buying separately an enclosure and a drive will yield better results.
About FS - if you want a sane native Linux FS which is at least readable from Windows, go with ext3 (and ext2 read driver whenever ext3 read driver is unavailable, I can't say what is current compatibility situation). For Vista leave a small FAT32 partition.