If you want a faster SSD, I'd go for the Crucial M4. However, it'll cost more (retail) and the 320 series is quite reliable.
Putting that much hardware in the PC with only a 20" display is...well...stupid, TBH. Since they have some extra money, you could easily bump up the display as well as the keyboard/mouse. I'd recommend a Dell Professional series monitor (P2412 or the like), as I have the P2411H (last year's model) and it's spectacular. It has a great design, has vibrant colors, a slightly matted display, and the stand is fully adjustable (even does 90 degree rotation from landscape to portrait).
Mouse and keyboard are personal preference, but if they type a lot a mechanical keyboard might be a good investment. Cherry MX Blue switches tend to be the best for typists, but are also present in a lot of gaming keyboards. As for a mouse, Logitech makes good stuff, but my personal favorite is the Cyborg RAT 7. It's fully customizeable and extremely comfortable.
Unless they are going to watercool (which takes a bit of know-how), you don't need to spend that much on a case. You could get similar performance from a Cooler Master HAF 932/942 or looks from a lower-priced Corsair/Antec/NZXT.
As for hard drives, you could wait if you wanted and just run everything off of the SSD for now. I'd imagine the 1.5TB will be mostly for data storage.
My sister is a professional photographer, her boyfriend does a lot of video shooting for weddings, personal events, etc, so they require a lot of hdd space.
Well I guess you can't really afford to wait a month then, huh?
He specified that he only wanted 20" displays.
Seems very small, especially if they're doing video/photo work; having a larger display really is a huge benefit for that kind of stuff. Take a look at Dell's Ultrasharp series - the IPS display has great colors, and I don't think its response time makes it too slow for gaming. Otherwise, I'd still recommend the Professional series.
Yes, they want it watercooled (don't worry, I know how to do it) I just need advice on which parts to get, as I've never done watercooling on a high-end computer.
I think watercooling their computer would be highly impractical, unless it's mostly for looks. Any reason as to why they (I'm assuming he) wants to?
Well, my sister told me she'd need the computer in around a month and a half's time, her current computer isn't too bad, but one month from now is just the projected buying time. (Waiting for HDD prices to go down..)
I'll ask him if he wants to upgrade to larger screens.
Well, the reasoning for the watercooling is 50/50 on practicality and looks. They want me to overclock their CPU to at least 4.7ghz, as he'll probably be editing video in one screen, and editing photos in the other.
And yes, you were correct with that assumption. It's just that my current build is watercooled (albeit old, probably around 2008?), he wants the look that I have going on right now, so I told him I'd do it for him, haha.