I'm pricing machines that will be primarily used for C# development using Visual Studio, and can't see a reason to pay $200 more for an Intel 2600 vs. a 2500. You gain 100MHz, 2MB of cache, and HT (this last one seems to be the selling point).
Is there something I'm missing that makes these worth the $200 extra? I'd much rather drop a portion of that into getting another hard drive and slapping the two of them in RAID 0. Seems like a better investment, and a quarter of the price.