Wanting to upgrade my PC

Most definitely it's the low end graphics card you're using. The R5 is a low end gpu and has only DDR3 VRAM. You're best bet is getting a better FASTER gpu, like an R7 280/290/290x or even an R9

What's my best bet with a price range of 250-280 and below?

You didn't list your power supply. To be honest, you might be a to get an RX 480 if ur lucky. But most is OOS everywhere now. I'd say either wait for in stock or go for an R9 380/380x

I have a power supply of 400W

Make? Model?

You need a quality 500W PSU MINIMUM for the R9 series

iBUYPOWER CA729T

Absolute junk unit....sorry don't mean to.be harsh. You DEFINITELY need a QUALITY brand psu. Corsair (not CX series), Thermaltake and FSP are all good brands.

Would the RM750 be considered a quality psu?

YES, definitely!! A extremely good psu...one of my favs. I own the rmx 850 myself

Then I guess that's what I'll invest in. However, I'm still going to need to upgrade my video card, right now I'm interested in the MSI Radeon R9 380, but don't know if I should get the 2gb or the 4gb.

Go for the 4GB. Almost all games today require 3-4 GB of VRAM to play efficiently.

I have the Gigabyte R9 390 8GB 512bit. Have yet to find a game I cannot dominate

Thanks so much. Is that all I need to upgrade in order to get great frames?

I would go for the RMx 750 or 850 Corsair PSU.

Yo answer your above question, no. you have an FX-4300 CPU. thats a very old cpu for todays games. if you planning to play stuff like BF4 and BF1, or any other brand new games with this build you are definitely going to need a faster CPU. I would recommend going to a Newer DDR3 motherboard with something like an FX8350 CPU and minimum 8GB RAM for todays games

See my build is this

i7-6700K
RMx 850W
R9 390 8GB
16GB DDR4 @ 2400

thats why i can "nuke" any game i play. your cpu is going to somewhat bottleneck these new graphics cards.

Sounds like I should just be buying a new PC in all.

Well it REALLY depends on what you want to play and at what resolution. If you just want to play average day games on decent setting a new GPU should be fine. BUT if youre wanting to actually get a hard core pc gaming system you are going to most likely look into getting a newer system. I'll tell ya what, tell me your budget and then i'll make a parts list for ya. Take into account if you want to reuse something from the old pc like a dvd drive or a hard drive, or if you want to simply sell the old computer in its entirety and put that money towards a new system

Basically, I'm just wanting to boost frames and run Arma at a high graphics quality, but it would be nice to be able to run a game such as space engineers. The most I'm looking to spend is 400/450 at this point. I'd like to keep my model and hard drive.

Alright. I can definitely do with those prices. Is that all I'm going to need? Well for what's in my price range, I guess.