You should be able to run two 3870x2's in Crossfire! Just make sure that you can meet the power requirements (you will have to use 4-6 pin of not overclocked / or / 2-six pin and 2-8 pin connectors). You better have over a 1000 watt PSU as well. I would think that since both the 3870 in SLI or One 3870x2 should be pretty comparable in speed...since you are talking two GPU's. You might find two 3870's cheaper than the one 3870x2 right now.