Budget gaming PC build $500-600

What would you recommend for a monitor?

A high end CRT if you're serious about CS:GO.

I just want to play it for fun just a good monitor for gaming

Woah, Woah man, you don't just play CS:GO for fun okay, that is what CoD is for.

You'll want to spend a bunch of money on a 144hz panel, but if you don't want to do that, this AOC monitor is pretty solid and low priced for an IPS panel.

http://www.newegg.com/Product/Product.aspx?Item=N82E16824160167&cm_re=AOC_22-_-24-160-167-_-Product

I can't play it serious with the pc I use atm. It gets a whole whopping 15fps even lower on the bigger maps

Should probably just shoot for the 1080p IPS panel then, or find a nice CRT on your local craigslist