The 2600K wouldnt do much better than the 2500K on Gaming or CS5, unless you do them at the same time (Hyper Threading). If this is a HTPC-type build, I would save $100 and get the 2500k.
You have great Upgradablilty with this build, but are you actually going to use it? How seriously do you wish to SLI in the future? If you dont SLI, you can save $70-80 on the power supply and then some more on the Motherboard. Also, you have picked a very high quality Board with the z77 sabertooth. This will give you great reliability and increased OC ability, but may not be required for your needs. Also, the benefits of z77 in general are questionable, and you could pick up a Asus p8z68 Pro for ~$150, which will be just as reliable and OC'able but would cost much less.
The bottle neck in this system is the graphics card. So If you do plan to play the newest games at 1080p and good detail levels, I would upgrade to a AMD 7870 or even a 7970 or 680 GTX. You can easily save enough money by getting the 2500k and z68 board to pay for the upgrade.
Thank you both. I think I'm going to go with the i5-2500k, lower profile ram, seasonic PSU, and updating to a 570 GTX. I think I got myself emotionally attached to the Sabretooth z77, so I'm going to keep that around and go to a 570 rather than a 580/680 (the prices I was finding on newegg for the 580 and 680 seem prohibitive, and while good graphics is important to me, i've been on a laptop so long that anything is better than where I'm at now).
Thank you SO much for your suggestions; this is great!