You really should't have these problems,. but indeed if you got very small mini-case cooling is more difficult, maybe try add a large fan to get more air flow,.
My former Graphics card was covered in dust so badly and still ran fine,. ;p
I can tell you i am def satisfied with my Cooler master tower stocked with 7 fans,.

@PSPKILLER
I have bought a compresed air can to keep up with the dust,. but aint worth the try,. dust does't come off and often the can sprays lliqued air and makes the dust even more sticky,.