Upgrading GPU (and PSU?)

Hi! Seeking advice for upgrading GPU, and probably PSU.
HP ENVY h8-1400z Desktop PC
• Windows 8 64
• AMD FX-6120 six-core processor [3.6GHz, 6MB L2/8MB L3 Cache]
• 6GB DDR3-1600MHz SDRAM [2 DIMMs]
• 1TB 7200 rpm SATA hard drive
• No secondary hard drive
• 1GB AMD Radeon HD 7570 [DVI, HDMI, DP, VGA adapter]
• 300W Power supply
• SuperMulti DVD Burner
• Premium Wireless-N LAN card and Bluetooth(R)(1x1)
• 15-in-1 memory card reader, 2 USB 2.0 (front), 2 USB 3.0 (top)
• No Additional Office Software
• No additional Security Software
• No TV Tuner
• Beats Audio (tm) -- integrated studio quality sound

This is what I have right now, with HP W2371d 23-inch Diagonal LED Backlit Monitor.

I play occasionally, usually real-time strategy games with the newest being Civ IV or V & Red Alert 3. My wife will also be doing a lot of graphically intensive work, such as editing photos and creating infographics.

What would you suggest I upgrade to in terms of GPU and potential PSU? Looking at no more than $500 for both. Hoping a potential solution will be relatively future-proof, but I'm not interested in SLI/Crossfire solutions.

Many thanks!