It depends on how big you want your pixels. Personally, I feel that 24" is about the limit for a 1920-wide monitor when used at normal monitor distances (1'-2'). Anything bigger than that and the pixels become big enough to be distracting. That is just my opinion though, others have different feelings.
For me, the main gains in going for a 2560-wide display are in productivity. I feel like I can manage a lot more windows doing a lot more different things with the extra desktop real estate. Gaming is also more immersive, but to properly game at 2560 requires a pretty beefy (and expensive) GPU setup.
I know that there's not a hard and fast answer buried in all of that, but that's because there really is no hard and fast answer. Bigger and more pixels is generally better, but you've gotta have the budget for it, both on the monitor and on the GPU side of the equation.
PS. I usually don't worry too much about pixel response time (what you see quoted in specs) because it is super easy for manufacturer's to bend the truth. Input processing latency is a far bigger issue IMHO, and you will only get measurements of that by reading a proper review.
LOL, you should see the thread I made about projectors...I basically decided I was going to do some long term planning and make a very good setup - even if takes 2 years to put aside the cash - so I'm trying to learn everything I can. I was thinking my goals are these:
1. Get a good monitor for regular computer use
2. Put together either a triple projector setup, or a triple monitor or LED TV setup for gaming.
It basically looks like it boils down to this:
3 Projectors: Pro : DLP is very good response time, projectors don't weigh much or take up a lot of physical space. Con: Very hard to set up, expensive, need blending software.
3 monitors or (for example) 40" LED TVs: Pro: Very easy to set up Con: Bezels are a pain, TVs take up space, weigh a lot, and even if monitors might lie about response times, TVs don't seem to even tell you anything at all, so I don't even know how to find out.
I know I'm all over the place, but a couple of weeks ago I happened upon a 480p projector that was $200 so I thought "oh- cool!", and I of course quickly learned that 480p is junk, so now I'm just carrying out the natural order of progression from that point of discovery.
I've been talking about it at
http://forums.anandtech.com/showthread.php?t=2270275 , if you had any interest in commenting.
And yea, I know a GTX 560 Ti won't cut it. As of right now I'd figure I would need a GTX 690 to run such a thing(as I don't have an SLI board), on the other hand, by this time next year, maybe what right now could only be though of as an HD 8970 or GTX 780 would be good enough to drive such a setup - I've taken that into account as well.
As for budget, yea this would all cost $3000 or $4000 easy, but I put together my current PC over a year for $800ish watching for deals when it could have cost $1200 bought all at once, I figure if it takes me 2-3 years to do the same for this, it's OK - it will get done. Though if I knew a bezeless 32 or 40" TV was coming, I'd wait for that.