here are my thoughts, for what they're worth. I faced a similar situation. I could have gotten a 2500 for $100 more than the 659. I looked at the specs, and the only advantage to the 2500 really, was a bit of power, THX, and HDMI. Untill they solidify HDMI (and I get HDTV) I think it's more fluff than anything. THX would be nice, but I don't really think it would add anything to the 659 exept $$. It's probably capable to be certified, hardware wise. The power difference is probably not noticable, unless you're trying to go deaf.
The 659 has many feature and connectivity advantages. I bought the 659, mainly due to the better features. And I got it for $369.