For a $150 max. budget, you'd pretty much be limited to choosing a 1080p monitor (can't fit a 1440p or 4K with that).
OPTION 1: Re-using your current IPS monitor for movies and buying a new monitor for gaming instead
Since you already have an IPS monitor (your current Acer H226HQLbid), how do you find gaming and/or watching movies on that specific monitor? If the current IPS monitor is acceptable to you for watching YouTube/NetFlix, as an alternative upgrade, you can re-use your current monitor as your secondary and, instead, get a faster/better 1080p ~75Hz w/ FreeSync TN-panel monitor to be used now as your primary monitor for gaming, since you are using an AMD R9 290 that supports FreeSync:
PCPartPicker part list /
Price breakdown by merchant
Monitor: AOC - G2460VQ6 24.0" 1920x1080 75Hz Monitor ($149.98 @ B&H)
Total: $149.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-07-19 13:20 EDT-0400
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OPTION 2: Buying a new secondary IPS monitor for watching YouTube/NetFlix
If you still prefer to use your current IPS monitor for gaming or if you feel the need for a bigger-sized 1080p screen for watching, then, as another option, you can get the slightly larger version (23") of your same IPS monitor:
PCPartPicker part list /
Price breakdown by merchant
Monitor: Acer - H236HLbid 23.0" 1920x1080 60Hz Monitor ($149.88 @ OutletPC)
Total: $149.88
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-07-19 13:28 EDT-0400
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If I was in your position with a limited $150 budget, I would choose option 1 so that the GPU (R9 290) would maximize its potential of delivering slightly higher fps than 60, as well as buttery-smooth frames (less lag, stutters) due to variable refresh rate the suggested AOC FreeSync monitor offers, that is, if your current Acer 21" IPS is good enough for you to watch movies.