What should I upgrade first for better framerate?

iTzShawn210

Honorable
Jun 26, 2013
3
0
10,520
All around, this computer aint cutting it. This build was shown in a video, promising great results for around 400$. At first, things ran fine, but now i get low fps in games that used to run fine. Ex: L4d2 ran fine with high settings. Now, i only get the same fps when there isnt much going on on-screen.

These are my specs

Operating System
Windows 8.1 Pro 64-bit
( I was using this when i first built my pc december 2014. I then went from win7 32 bit back to win 8 after not seeing improvement)
CPU
Intel Pentium G620 @ 2.60GHz (considering replacing with an i5)
Sandy Bridge 32nm Technology
RAM
4.00GB Single-Channel DDR3 @ 532MHz (8-8-8-20)
Motherboard
ASRock H61M-VG3 (CPUSocket)
Graphics
DTV-C7V (1920x1080@60Hz)
2048MB ATI AMD Radeon HD 6670 (XFX Pine Group)
Storage
465GB Western Digital WDC WD5000AAKX-753CA1 (SATA)

Now im wondering what can i do to play my games smoothly. There isnt exactly a budget, but i'm not trying to buy 1000$ parts.

What should be replaced? Or anything added?