Leaps-from-Shadows :
At the level of horsepower we're talking about here, the video card is more important.
If the 2500K allows you to get a GTX 570 instead of a GTX 560 Ti, get the 2500K. You'll be glad you did.
And as far as more cores = future proof ... we've had multi-core systems for how long and yet the vast majority of our games still take advantage of only one or maybe two cores? Basically, you'll be upgrading to an entirely new system before you ever have a chance to regret getting a 2500K instead of a 2600K.
Really? That's an amazing prediction. I've been running my same gaming system for over 4 years now and don't plan to upgrade till BD or SB-E.
It's great if your JUST a gamer like most kids on here. However if you use REAL applications, the hyperthreading is definitely worth the extra money, your getting basically 4 more core. Oh and just so you know, there are A LOT of games that use 3-4 cores and some use 5-6 cores. Don't be left behind behind because your taking a gamers point of view.
If you get the 2600(k), you won't have any reason to upgrade for 3-4 years, where as if you go with the 2500K, you'll feel left behind when 8 core BD's and SB-E's start rolling out in just a few months.
If your strictly a gamer and surfer, then by all means save the $100 toward a better VC and get the 2500K.