Would you consider getting something other than an Alienware? There are a couple of vendors like Asus and Clevo/Sager that offer systems with the new HD 5870 that would honestly be better options. If so take a look at the Asus G73 and the Sager NP 8760.
On to your actual question. If you look at the benchmarks on Notebookcheck.net, a single GTX 260M gets out performed by a single 4870 anywhere from as little as 3 or 4 FPS to as much as 20 depending on the game and settings. Dual GTX 260M's should be able to beat out a single 4870, but only in games that can take advantage of a dual card setup (these are becoming more and more popular).
However, if you intend on doing other things other then gaming, like CPU intensive apps, then you may want to consider the i5-540M based system. The i5-540 at 2.53ghz is as fast or faster than a Core2Duo clocked at 2.8ghz.