The i7-2600 has hyperthreading, which is worthwhile if you're going to be streaming and/or doing videos too.
If you're primarily gaming and not doing the other stuff, a i5-2500 is about $100 cheaper, and about the same performance.

The K on the end stands for an unlocked multiplier, meaning you can overclock it fairly easily... just want a decent (read non-stock) cooler on the processor.
I went with a Corsair A-70 which was around $50 Canadian.
If you're not interested in overclocking the CPU, the non-K processors are a bit cheaper and the stock coolers should be fine.

The motherboard, processor and ram will be different... and you'll need to upgrade those together.
You might need to upgrade the power supply too, but not necessarily.
Everything else plugs into the motherboard, and is extremely easy to do (check a few YouTube videos); you can of course have a computer store do it all for you too.

Memory Express (Vancouver, Calgary, Edmonton, Winnipeg) has an IPR program (instant-product-replacement) which is a very nice warranty option... if you buy the parts at whatever store and have that store do the installation, you might have the option of a nice warranty too.
Basically, anything goes wrong with the part and they replace it with a brand new one (had one SSD replaced, and one monitor had dead pixels) not a refurbished product.
Doing it yourself will be cheaper, but a warranty is nice too.