I can play all jour games un my 500mb GPU and 4 GB RAM
So for you its no problem!

 

What would be a great gpu replacement with that power supply?

 

I doubt that psu will push anywhere near 500w before failing

 

R9 390 or GTX 970 can both be picked up for around the $300 mark on newegg.

GTX 970 uses around 170-180W under load, depending on which version you end up finding. Assuming the rest of your system uses around 250W (I.e. Assuming you're not overclocking) then you should be around the 420-450W range, which should be safe enough on a cheap PSU brand, although I agree with @Joe88 that you don't want to push it anywhere near the claimed rating of 500W. Cheap PSUs rarely hit their claimed rating before exploding (more smoke than fire) and brandless PSUs are fire hazards waiting to happen.

You might be safer getting a graphics card one step lower like the R9 380 or GTX 960 and using the $70-100 saved from the budget to getting a PSU from a decent brand like Seasonic. You won't need more than 500-600W, but decent brands can usually go 100% safely and even sometimes 120% safely (but not recommended), and tend to have much higher energy efficiency (gold rated or higher).

 

If i were you, I'd get one of those Haswell pentiums with a reasonable motherboard and a so-so graphics card. [Short term solution] (maybe you could sell some old stuff)
Long term solution, invest in a good, robust gpu first, then the others.

But since you wanna play at 720p, you might as well go with the short term solution i just mentioned. A GTX 950 card would be more than enough i think.
eg. Intel Pentium G3258
ASRock B85M-ITX
MSI GTX 950 2GB GDDR5
Total: $284 (prices taken from newegg)
Use the difference for some DDR3 ram

 

bump, well according to wikipedia I cannot upgrade the gpu any higher than the 4670, unless they are wrong about that. I also meant this for older gaming not newer stuff I'll say pre 2012 games. Might want to switch out the gpu soon not sure but what would be the nvidia gpu equivalent to the hd radeon 4670 would it be worth it?