What's your current system specs? This will help with the choice between a 1080p or 1440p, between 100, 120, 144Hz or higher.

Also just a tip, the whole 1-5ms hardly matters at all. Sure if you're a professional I guess you would need that fraction of a second but casual or competitive not on a pro-scene, you won't really need that 1ms stuff as that limits you to a TN panel which isn't as crisp and beautiful colour wise or have good viewing angles compared to an IPS monitor which are 5ms and some are like 7/8ms. Higher refresh rate such as 144Hz is what will benefit you more if your PC can reach that FPS and remain stable. There's a huge difference jumping from 60 Hz to 144 Hz. I wish GPUs could run 4k at 144Hz or higher as switching from my 1440p 144hz for competitive FPS, over to 4k 60Hz for casual gaming is such a strain on my eyes at times.

my specs are:
Intel Core i5-6400 CPU MSI R9 380 4GB GPU
yeah thats what my brother told me is that the ms response time really doesn't matter

What exactly do you use your PC for? What games and programs/applications do you use?
What kind of monitor would you prefer, higher resolution or refresh rate, or ultrawide?

$800 is a pretty big budget, it'd be much wiser to spend some money on system upgrades, and the rest on a new monitor IMO. A low end i5 and low end GPU paired with an $800 monitor wouldn't be a great idea.