I also might add a hauppauge HD PVR in the near future
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More aboutdual 6870s
THe 6870 only draws 150 watts, so your PSU should be plenty for 2 of them. I cant find a reveiw that talks about the amperage requirments of the 6870. So you will need to check what the max amperage is of your PSU and what the 6870 might draw.
The 6870s CF require around 55-60 amps on the 12v rails and minnimum 700W PSU. So you should be good to go.
Edit: Your PSU has a single RAIL at 62AMPs. You should be fine! Have fun.