Buy a fast GPU for $300 290 series @ $399 or 280/7970 @ $309 or get the CPU setup I put below. Personally I would get the GPU setup, then get hold that extra $100 and wait until I had $200 more to get the mobo / cpu
If you want to work it, you will need a better heatsink / fan. Take a look at my case pics to get an idea. What are your benches right now in games? In my opinion since $300 - $400 is low for a new setup but you might be able to do this and STILL use your ram:
Intel i5 4670k $225
ASRock Z87 Pro4 - $125
This gets you to $340
Then if you want to go crazy
8GB Corsair Vengeance for $75
Another thing I would do is also get a CPU cooler:
You can get a Thermalright or Cooler MAster 212 EVO for $35
Another few things, I would recommend getting a nice mouse later on like my deathadder 2013.
This setup will be fine with your current PSU.
Another few noteworthy things I would get
GPU: I cannot stress how important it is for you to get a new GPU...... If you can get the GPU first and the cpu / mobo later. Your current CPU is what people used a GTX 460 and earlier gpu with. I know people who can get the Phenom X4 955 to 3.8ghz it is a similar chip when O/C as an intel 2500k. What does this mean?
It means, you are really lacking in GPU performance. Splurge. Look at me I have an i7-930 @ 4.3Ghz and bought Titans (those were $1k a pop).... people normally would say to themselves "Why in the world" ( would rather say something else but I got a warning for using acronyms)...
So GPU options:
AMD: Radeon Gold award comes with 3 free games, and you can get RAMDisk ( get it for $9 bucks instead of $19.)
290X fall into $$550 series which equates to a GTX 780 (costs $$729)
290 series for $399 falls into the GTX 770 series
280X is $300 to $200 and is like a GTX 760
Anything below this in my opinion is not worth it.
You can get the 290 series for $399 with BF4 free ( $60 value), 280X for $300
The difference you are probably asking since the 7000 series comes with 3 free games and the R9 series comes with only BF4
the MSI 280x is basically a 7970 with tweaks and higher clocks and slightly faster. So if you find a MUCH cheaper 7970 and want the 3 free games (not BF4 get that)
Personally I would go for the 280X
if you like freebies the R7 260X gets 2 free games( dont buy it spend more for the newer cards)