Well. I don't know too much about the GTX 285m, but my little bit of research shows me it's a little worse than the GTX 580m. (Really? 3 series of GPUs and a minimal performance increase? Come on.)
Anyway, the 6990m doesn't quite have the performance of the 580m. It's close, but not quite. Crossfire scales very well these days though, and that would give the xfire 6990's a boost. But the performance increase might be hit or miss. The processor in the new laptop is by far better, but considering this laptop probably costs $2000+++, The performance increase you get out of it probably won't be worth it to you.
Makes me wonder though, is anything really slow enough on that laptop to even consider an upgrade anyway?