What sort of return (block/week) should I expect on the 5870?
11 BTC per day in pool before next difficulty step (expected in 3-5 days). Then 30% less.
Can I run cpu in pool as well without degrading my gpu mining performance?
There is no point in doing so. Overclocking 5870 just a 1-3% will be more noticeable than all your CPU power.
5870 crunches at 320-350 Mh/s depending on core frequency and your CPU will give only 2.4-4.5 Mh/s.