The Graphics Card I have is Sapphire Radeon 7950 4L with a 925 core clock. I was wondering if it is safe to overclock the gpu while I have a locked cpu (intel i7 3770) and if so, what is a safe rate for this gpu?
The "safe rate" varies from card to card so that's something that you will need to figure out with experimentation. The fact that your CPU is locked will have no effect on your GPU's ability to overclock because they are separate processing units.
Don't worry about your CPU being locked, it has no effect on the GPU clock speeds.
Here is what they got on TechPowerUp with their overclocking of six different 7950s. The maximum overclocks they got with default fan and voltage settings range from 1000 MHz to 1210 MHz. It looks like somewhere around 1100 MHz should be a top-end target overclock to shoot for. http://www.techpowerup.com/reviews/HIS/HD_7950_X2_Boost...