I have the Apple 2TB Time Capsule for Time Machine backups. I like it because it’s dead-simple to configure and just works (especially when I’m connecting to my iMac from my MacBook Air while on the road). I use Time Machine as a first line of defense, but not permanent backup. The Time Machine overwrites the oldest data on the drive when it fills up to make room for new data, so don’t think of Time Machine as a backup as much as just a quick rescue.

For permanent backups, I use external 1TB Seagate USB drives. I typically back up new data once per month or so, and store them off site. I like to stick with 1TB drives because I figure if one goes, I’ll lose less data. Plus, it’s easier to keep track of what’s on them. I tend to keep personal data like photos, documents and music on one, and use another for client work. Once they’re nearly full, I label the drive and grab a new one. With video, you may opt for a larger 2 or 3TB drive though. I pretty much keep everything on my iMac, but it sounds like you have a LOT more photos and video than I do.

I haven’t used a scratch disk since the days of Mac OS 9. OS X is extremely smart when it comes to memory management and disk defragging. For Photoshop work, you simply don’t need a scratch disk anymore. The amount of RAM and speed of the video card and processors are more than adequate for all but the most extreme case use. I can’t speak intelligently about serious video work though, as my video work is limited to quickie iMovies of my son, etc. No serious video work at all.

There’s probably no right or wrong opinion on this subject, other than do NOT trust Time Machine as your only form of backup, because it simply isn’t a permanent backup system (whether you use Apple’s Time Capsule or any other external drive with it). Eventually, that drive will fill up and Time Machine will happily overwrite your backups. You just have to find a system that you’re comfortable with.