The speed of the card affects the number of frames you can shoot per second; everything else being equal. Also, the quality of the picture is affected by the format used - RAW Vs JPEG - JPEG pics are 'processed by the camera's software and then stored/displayed as an image; whereas RAW is stored in pixel form and needs to be processed by the photographer much like a negative in film format.
The photographer has much more latitude working with the RAW format. Hope this helps.
DSLRs and HD video cams compress their video signals to set frame rates that usually conform to PAL or NTSC standards. As long as your SD card's write speed tackles the data transfer rate of your video signal out of the camera, then there will be no problem. If not, it will simply stop recording or, depending on the camera, result in very visible frame drops - your signal will be unusable.
JPEG is a compression format for still images, and RAW also refers to still image work. Most HD video is captured with H.264, AVCHD, an MPEG2 variant etc.. These capture streams can be transcoded to video streams with more color latitude for post-processing, like cineform, lagarith etc..
Luckily, some cheap SD cards have write speeds that are much higher than their classification and more than fulfill the requirement for most cameras. See here for more some tests with cheap class 2 cards on a Canon 550d/Rebel T2i shooting full HD.
I have 2 different speed SD cards and what i see on the slower one is a more noisy video. Also with more banding on surfaces with grading.
What i think is that is because of the VBR (Variable Bit Rate) of the Canon DSLR video.
Also, fast moving video affects the bit rate. (Correct me if I'm wrong)
I'm waiting the shipment of a 45 mb/s SD card. As soon as I get it I'll do a comparison video. If you want to keep track of what I'm doing check out my blog "Primer Corte" in a couple of weeks from now. pedrobros.com
Camera I use: Canon t2i/550D
Cards used so far: Generic 8GB Class 4 SD, ¿?MB/S, Transcend 16GB SDHC Class 10, 20MB/S
Card I'm buying for the test: Sandisk Extreme Pro SDHC UHS-I 16GB, 45MB/S