I'm going to build a pc and it's mainly for multimedia applications like AutoCad,Photoshop, and some video encoding, it's also for gaming.
so do i need an i5-2500k or im good with just a 2400 for my needs and also im also considering to change the gpu initially from a palit gtx 560ti sonic to a Sapphire Radeon HD 6870 would this change have a great affect on the task at hand?
if you want the best then the i5 2500k is worth it once you OC it. Cuda is helpful is some programs and you'd have to check which ones support them but the cpu can still do the tasks if you get an AMD gpu.
the 6870 is very good performance for the price in gaming tho. You might also consider the 560 (non ti) for simular performance while supporting cuda, it ussually costs slightly more than the 6870 but might be worth it for your needs.
opengl is a graphic standard library, its used to code for rendering. It is not the same as CUDA which is a implementation from nvidia to do computing that does not necessary have to be gpu based on the gpu.
Some things the gpu is faster than the cpu so CUDA allows for code to be executed on the gpu if the program chooses to do so. Not all programs support cuda but is does work for some. AMD has AMD Stream which is their competing technology which has less support.