Don't know what the better bang for the bug is between those 2. I have not seen benchmarks and prices.
AS for you questions.
1; maybe. If it's a good PSU - Preferably gold validated - you can properly still use it with that new card. If you don't pay your own power bill, it's gonna do fine.... if it's of good quality that is. Any cheap brand and i'll trow that thing out.
2: It's all about the game engine. Engines like Cryengine and it's twin Dunia Engine runs best on Nvidia.
Where the source engine games run better on ATI/AMD.
My boss at work thinks I should wait for the GTX 960 come out, but I can get a Radeon 290X for around a hundred dollars less than a new 960. Would I regret going to a Radeon?
I'm still unsure about which direction to go. I want some awesome performance, and my i5 4690K is on the way, so I guess it's time to match it with a good GPU. My PSU is a Cooler Master, so I have no clue if that's considered a "good" PSU. I might go up to an 850 Watt Corsair.
I have a Corsair HX850 on the way. It's gold rated, and I got it for $40. I'm so cheap . But it looks like I will be going with either a R9 280X or R9 290. I guess it really doesn't matter, because the most intensive game I want to run is DayZ Standalone. Is it worth it to get a RAM Fan? I found a kingston that would work really well, and I also am looking to get some really quiet fans..... Any suggestions?