I have cleaned my GTX 650 ti and GT 430 multiple times. The fans are prone to dusts.
You may clean it with cotton swab (http://assets9.heart.co.uk/2012/41/cotton-buds-1349948617-view-0.jpg) carefully.
Step by step?
Before you begin, you should collect the proper tools and know how to use them. You will need different sized screwdrivers to help with the disassembly (most modern cards use three different sizes of Phillips-head screws), plus cleaning materials such as canned air, alcohol, and cotton swabs. I like to use a small brush to wipe away additional dust and old thermal interface material but that is optional for you.
1. Open the case. Take off the plug on the power supply, behind your case.
2. Unscrew the 270x.
3. Carefully and slowly pull the graphics card from the PCIE
4. You canned air, or if you don't have, you may blow away the dusts on the microchips or on the top, sides of your graphics card.
5. Use cotton swabs to wipe off the dusts on the fan. Use of multiple swabs is recommended. Once the first swab is filled with dust, use a second one, the third one, and so on.
6. Blow away the remaining dusts on the graphics card.
7. Return the graphics card and plug it to the motherboard's PCIE. Return the screw
Note. Do not use water to clean the Gold Contacts. See this: (http://4.bp.blogspot.com/_g63vUhYy6Cs/TBoI1BubipI/AAAAAAAAABo/k4shbJOcbbU/s1600/video-card-integrated2.jpg). Alcohol is used because alcohol dries faster than water.
Hope that helps. There may be more ways to clean it. But that's the simplest one for the first timers.