It depends.
Is image quality your biggest concern? Do you need the resolution (say for viewing full 1920x1080 content)? Is 24" a bit too large for you (I know several people have said its just too big to sit in front of)? Have you ever seen a monitor with a glossy screen?
The NEC will undoubetdly have better image quality. I'm sure you're aware of the resolution difference. You might view it as a good thing, maybe a bad thing (good if you don't want to have to upgrade as often to be able to play games at native resolution, bad if the extra resolution comes in handy for whatever you do).
From what I've gathered, it would be a better monitor for gaming. Some users have reported some type of input lag with the 2405, and also the NEC has better response time as you've noted.
If possible, I'd recommend checking out a monitor with a glossy screen in a store to see if its something you could deal with (more susceptible to glare). Some people are really bothered by it (and nearly every review I've seen puts this feature as a negative against the monitor), and others love it. Keep in mind your setup at home, so if you have a large window right behind you, it might not be the best.
I would say, that from what I've heard about the NEC, that it'd be worth the downgrade for you. The lower resolution seems to be a benefit for gamers so that they don't have to upgrade as much (but if you do it anyways, it might be a moot point). Also, the response time is second to none in LCDs. Couple that with the superb picture quality, and it should make for one sweet gaming screen.
I believe there are supposed to be a new large panel that should be close to the NEC, but in I think 23" size sometime. Not much more information to offer than that, but something to consider if you really like the extra size and/or resolution. You might PM the user Zebo on here, as he seems to know a lot about anything like that.