Hey,
I'm not an expert here, but I run 2 6950 cards in my rig things I know:
You shouldn't ideally go over 80c, 85c from what I understand these should be your max temp ideally, you can go over but once they hit 90-100 they start to cook complete, personally I run mine around 69-73 but I run my machine with the fan on high for the sake of keeping the temps low at all times.
You should actually be unclocking the memory, memory isn't used at all, I've not worked out the science of this but basically mine without the memory underclock wouldn't mine in a stable manner, and it's also the memory that adds a lot of heat, I underclock mine to 795Mhz.
My main thought is, 300Mh/s seems low for that card (you can guestimate what you should be doing from here
https://en.bitcoin.it/wiki/Mining_hardware_comparison), maybe you should try a better miner like CGminer or look to use Linux rather than Windows if your in this for the real mining experience.
