Upgrade advice/ Why does my PC suck so bad

Robomoo

Honorable
Jun 13, 2013
29
0
10,530
My system:
(about a year old)
AMD FX(tm) -6100 six core processor 3.3ghz
4GB RAM
500GBHDD

(fairly new)
Radeon HD 7870
XFX PRO550W psu

Some games (e.g planetside 2) I can't even get playable framerates for some reason. Then sometimes in stressful applications it'll do a black screen and crash. Dunno why this is. Advice please.