so lets see
If I just wanted to upgrade to 27" (1080p), then the options are endless and fairly cheap. Here's a BenQ w/ 1ms latency for $258.
BTW, my current monitor has 5ms and I have no complaints. I'd go w/ something like this maybe if all I cared about was a bigger screen.
Here's a 27" (1080p) Samsung curved monitor.
This one seems less game-y, though. 4ms latency.
Now for $377 I could get a 24" (1080p) Acer Predator.
It's only an inch bigger than what I have now, but it's got G-Sync and runs at 144hz refresh rather than 60. It's got 1ms latency as well. This seems like a decent compromise... *slightly* bigger screen and G-Sync. EDIT: PC Gamer's Guide To Monitor Technology also says that anything over 24" is going to start to look bad at 1080p vs how it looks on a 24" and under screen.
For $400 there's a 27" 1440p Asus.
This is not necessarily a "gaming monitor." I'm not sure I'm sold on needing a 1440p now. From what I've researched the big plus is having more "real estate," but I mean... I already have plenty. On all 3 of my PC's the only thing on the desktop is literally a trashcan.
Finally at $500 I could go with a 27" Philips (1080p) w/ G-Sync.
This would probably feel like the biggest upgrade to me as it's (A) much bigger than my 23" that I have now, and (B) has G-Sync which is fancy and new. $500 would really be pushing the ceiling of what I'd want to spend on a new monitor tho... EDIT: And maybe a bad idea to do 1080p at 27"...