As far as good GPU goes, I'd look at the RX 460. If your budget is 150, you should be able to afford the 4gb variant. At this level though, the difference between 4gb and 2gb vram is mostly only noticeable in poorly designed games or games with poorly designed mods. So if you want to save 15 bucks, the 2gb variant is not a bad choice.
The RX 460 also has minimal power requirements; it might run just fine on a 300W PSU from a prebuilt, but I would not recommend it. It *should* work (system load power should be in the 260-280W range), but the PSUs in prebuilts aren't always matched to what the sticker on them says. Basically, the PSU is one of the easiest places for an OEM to save a few bucks and increase their margin. They put in the cheapest piece they can guarantee will work with what they specced the computer to be. Sometimes, due to the way manufacturing works, it's cheaper to include a higher wattage PSU, sometimes not. Other times it cheaper to include a PSU that says 300W but only provides 250 cuz that's all the system needs, etc.