So I've been mining with my 6950 and 4850 for about a month now and just recently I noticed that my cpu usage is at 50% when mining.

If I unplug the cable to my 4850 and just mine with my 6950 the cpu usage is ~0%.

I quickly found that I was not the only person having this problem as seen in this thread: http://bitcointalk.org/index.php?topic=4446.0
Unfortunately there doesn't seem to be a definitive answer as to how to fix this problem.

My electricity rate is pretty cheap (6.62¢ /kWh) but I'm wondering if it's even worth mining with my 4850 as I am paying to power it and half my cpu for ~70000 kH/s.

Relevant specs:
poclbm 2011-03-25
Windows 7 Professional x64
AMD Stream 2.2
Catalyst 11.3

AMD 6950 2GB unlocked shaders
AMD 4850
Intel i5 2500k

Any help is appreciated.

You should drop the 4850. 4xxx cards have poor hash to power consumption ratio. Even with your electricty cost I think you're actually losing money on the 4850

Out of the top of my head:

At 70 Mh/s you have an average production time of 65 days, 13 hours, 57 minutes per block with this weeks difficulty. Assuming it remains at that level (which it won't), and your card is about 200W on load, now let's be nice and say 150, you'd be at 3.6 kW a day or 234 kW per block. That's $15 of electricity per block. Well, I'll be damned, that's some cheap juice you got there.