According to this link, (refer to this link from awesome user ceewi1 to check the best PSU for your video card) yeah your PSU can handle 2 x HD3870 since they would only need 550W. What I would need to know is how much +12V amperage it has.
The eXtremePower 650W is based on a 500W Seventeam PSU, which is rated for a total +12V@30A. It's not something I'd be inclined to trust with a pair of 3870s long term.