There are quite a few ways you could go here... I suspect you don't actually need more RAM yet; 6 to 8GB is probably the sweet spot if you aren't doing anything super intense. Your power supply seems fine, and the processor is still decent enough—you might even be able to squeeze some extra performance out of it with an overclock. You could try something like this:
1 x OCZ SSD ARC 100 240GB, OCZ Barefoot 3 M10 Controller 1 x Gigabyte GTX960 OC, 4GB GDDR5 128bit, GV-N960OC-4GD 1 x Zalman CNPS10X OPTIMA, CPU cooler. FAN: 120mm, fan speed:1000rpm 1 x ASUS M5A97 R2.0, AM3,AM3+, AMD 970, SB950, BUS 5200 MHz, DDR3
$1152 ADM
I added a cooler in case you're stuck with a stock one if you want to overclock. If the CPU ends up being an issue, maybe you could swap to an FX 8320E for about $367 once you sell the GPU, the motherboard, and whatever is left from those $117 pieces; then you could easily push it past 4GHz.
Or perhaps:
1 x OCZ SSD ARC 100 240GB, OCZ Barefoot 3 M10 Controller 1 x ASUS B85M Pro3, S 1150, Supports 4th Gen Intel Core i7 / i5 1 x Intel Core i5-4460 (3.20GHz,1MB,6MB,84 W,1150) Box 1 x MSI GTX950 2GB GDDR5, (GTX 950 2GD5 OC), 2xDVI,HDMI, 1xDP
$1373 ADM
The graphics card is weaker here, but you get both a new CPU and a whole new platform at once. You might be able to shave off some cost if you opt for a smaller 120GB SSD instead, assuming that's enough, or just stick with a 1TB HDD.
Just a heads-up, any programs already installed in Program Files won't run on 64-bit macOS, though your standard files like photos, music, or movies should be totally fine.
Kingston DDR3 HyperX Fury, 1866MHz, 8GB Black HX318C10FB/8 Patriot SSD Blaze R545/W430, 120GB, 7mm, 2.5" AMD CPU Kaveri Athlon x4 840 (3.1GHz,4MB,65W,FM2+) box ASUS Motherboard AMD A68H (A68HM-P33_V2) NVIDIA GeForce GTX 750 ti 2GB/128bit EVGA Hexa plus 400 w MS Knight MS KNIGHT case without PSU
Are you planning on hammering the RAM and CPU into the motherboard with a hammer? 😬 That processor won't fit that board, and it doesn't support DDR4 anyway. Plus, you're probably going to need a new power supply. Is this build meant for gaming?
2 x DDR3 2048MB ( 2GB ) Kingston, 1333MHz, Value 1 x XFX R9-280X-TDFD, DD Radeon R9 280X 1000M Boost 3GB DDR5 1 x Intel Core i5-4460 (3.20GHz,1MB,6MB,84 W,1150) Box 1 x Fortron 500W, Hyper, PPA5005003, 12cm fan 1 x Asrock B85M Pro3, S 1150, Supports 4th Gen Intel Core i7 / i5
For $83, you’re looking at a machine that’s maybe three times faster... I mean, it's a pretty old platform, but you could probably upgrade it using some used parts—grab an extra 2GB of RAM, a much better GPU, and maybe a quad-core chip for around 300-$117. You'd likely need to swap out the power supply too, though.
I guess you could toss a GF 6200 into the PC_1 and end up with 1.75GB of RAM if you use three sticks, but honestly, it probably won't make much of a difference.
65°C is actually pretty chilly for a GPU; I wouldn't sweat it unless you see it creeping past 90 😉
The whole point of side intake is to give the graphics card some breathing room, but since your other components aren't running hot, I guess adding one more 120mm fan up front should be plenty.
The performance is still about $30 higher... the integrated graphics on this processor are actually more powerful than all those cheap cards up to $167, so it should handle everything just fine, except maybe heavy gaming on max settings. You can always grab a dedicated GPU later and just disable the onboard one 😉
The second option is definitely better. I’ve put together a slightly upgraded version here (better case, power supply, and drive, plus an extra cooler):
Intel Core i5 Desktop PC GIGABYTE Main Board Desktop AMD A68 (GA-F2A68HM-S1) Fortron 400W Power Supply Corsair Vengeance DDR3 RAM, 1866MHz, 8GB Black HX318C10FB/8 Zalman micro tower case black, ZAL-ZM-T1 PLUS Seagate Barracuda 1TB, 64MB, 7200rpm LC Power CPU Cooler LC-CC-95 - 90mm Intel/AMD