That depends on your requirements. However, for mixed usage, I would still recommend getting a 1440p display with 144 Hz refresh rate for sharper visuals. As far as gaming is concerned, the RTX 2070 and Ryzen 7 2700x would efficiently run most of the titles at 60 to 80 fps. So a 144Hz monitor would be an ideal choice according to me.
Moreover, there are several options for panels in the market such as IPS, VA, and TN. In all of them, IPS panels are the best, known for thier vibrant colours and wide viewing angles. VA panels are quite similar to IPS, but not best. Samsung manufactures most of the VA panels in the market. I would not recommend TN since they feature inferior viewing angles (they are good actually, but I won't recommend if you care about colours and viewing angles). So, IPS panels are the best overall. Now if we look at the market, an IPS 1440p panel with 144 Hz refresh rate usually cost around $600. I know that's way above your budget. Although here are my recommendations.
Recommendation for 1440p 144Hz IPS:
• ASUS MG279Q FreeSync Monitor ($623)
Recommendation for 1440p 144Hz VA:
• Samsung LC27JG50QQNZA Series FreeSync Curved Gaming Monitor ($399)
Recommendation for 1440p 144Hz TN:
• AOC Agon AG271QX Esports Gaming Monitor ($390)
Recommendations for 1080p 240Hz:
• Alienware 25 Gaming Monitor - AW2518Hf ($336)
In case if you decide to get a 1080p 240Hz gaming monitor, then Alienware AW2518Hf is the best option.
Again, it's up to you. So, figure out your requirements accurately and then decide.