318? I'd call it close enough for government job, would you agree? ^^
You might try OC'ing it a bit, just because it's a decent card you got there.
These paramters are used for fine-tuning the miner to your card.
The -w parameter is the work size which can range from 64 to 256. Generally, for a 6950 the optimal value is 128.
-v enables vectors. You DO want to do this.
-f is for tuning the mining aggression (basically, the load on your card); the LOWER walue here, the more aggressive the mining.
FYI, GUIminer is but a frontend - the actual miner (down below the pretty gui) is, in your case, poclbm.
Below you can see the current source code for poclbm, all possible parameters, their default values and meaning are explained there:https://github.com/m0mchil/poclbm/blob/076f85c63d1ce21c81695eed9b60eb778a0f1ade/poclbm.py
PS. In your first post, you were like "i don't want to mess too much"
After a while you added the "-v -f30 -w128" string
Dude, by this rate tomorrow you'll be building a dedicated mining rig