Seeking help upgrading from MSI Radeon HD5450 to something better for gaming, Budget around +/- $150

[Xi57]

APPROXIMATE PURCHASE DATE: Within the next week hopefully?
BUDGET RANGE: +/- $150, possibly higher, depends on if it's really worth it.
USAGE FROM MOST TO LEAST IMPORTANT: Gaming (Dayz Standalone, Arma 3, etc.), Audio engineering (Ableton, Reason), Streaming.

CURRENT GPU AND POWER SUPPLY: CORSAIR CX-M series CX750M, 750W, 80 PLUS BRONZE - ATX12V & EPS12V Modular PSU

OTHER RELEVANT SYSTEM SPECS: Manufacturer: Systemax
Model: SYX-760GM-E51
Processor: AMD Phenom II X4 965 3.40 GHz
Installed Memory (RAM): 8.00 GB
System type: 64-bit Operating System
Monitor: Samsung 23.6" Widescreen LED Monitor HDMI (S24D590PL)

PREFERRED WEBSITE(S): N/A
COUNTRY OF ORIGIN: USA
PARTS PREFERENCES: Whichever makes transition simpler.
OVERCLOCKING: Idk what this is.
SLI OR CROSSFIRE: Idk what this is either.
MONITOR RESOLUTION: 1920x1080
ADDITIONAL COMMENTS: I'm concerned on whether or not it will fit into my current tower? Ordering something and it not fitting worries me :/

I'd really appreciate your help on figuring out what would be the best option! Again as I said before, I'm concerned on whether or not it will actually fit. If there's any additional info you need on my end let me know. Thanks in advance for the help!

 

[clutchc]

At stock, the Ph II X4 965 can just about keep up with a GTX 960 or HD-280/285/R9-380 max @ 1080p rez.
But to stay within your budget, you'll be looking at the GTX 950 or R7-370.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20600582123%20600565503&IsNodeId=1

 

[cdabc123]

used gtx 960

 

[Martell1977]

Wait for June 28th, AMD will be releasing the 480, 470 and 460. The 470 should be in your price range and use a lot less power. We don't have performance numbers yet but the performance is thought to be around the R9 380.