I am looking to buy an external hard drive to use with both Time Machine for the mac and just regular backups for my Windows PC. Is it worth it to get one that has Firewire 800 (for video editing)?

Are you interested in having redundancy? If so, then I'll suggest the Guardian Maximus from NewerTech. You can buy it with or without drives. It's super easy to insert your own (identical) drives and it handles the RAID1 automatically. USB, Firewire400 and Firewire800 interfaces.

You can format the drive however you like, HFS+, NTFS, etc. I'm not positive, but I believe Time Machine needs an HFS+ disk when connected locally. If you're going to do Time Machine over SMB then I believe you can use an NTFS volume on a Windows share.

If you're not looking for redundancy, then there's not likely going to be a huge difference between external cases that you can outfit with your own drive. You'll be looking at things like interface choice and style mostly. If choosing Firewire you might want to look out for an Oxford Chipset which everyone seems to recommend. I'd probably just pick up a cheap 1TB Western Digital MyBook for single-drive though. Right now, at least here in Canada, you can pick them up cheaper than buying a bare drive of the same capacity.