For the sake of the $20-30 I recommend getting the 2500k right away. Why spend so much on a cpu that is so close to your goal anyway? You will basically have to spend twice the money to get a minimal upgrade, since you don't plan to overclock anyway. Also the stock cooler is fine if you don't plan on overclocking, and the money saved right there is enough to get the 2500k right away.
Im not sure if you need USB 3.0 on your case or not, since your motherboard has a few 3.0 ports on it anyway. If you can do with just the USB 3.0 ports on your motherboard, then this case IMO is a better overall buy. http://us.ncix.com/products/?sku=57910 (49.99 after MIR, and free shipping) There are a TON of options in this case, just check out the specs. Also read the customer reviews while your at it.
PS: IF you do plan to overclock the 2500k and/or your video card, then you may want to look at a PSU in the 600-700w range just to give yourself some room to work with. Just my opinion, but I would probally look at something like this: http://www.newegg.com/Product/Product.aspx?Item=N82E168... Then if you grab a better vid card, or decide to go crossfire, or if you end up wanting to overclock you wouldn't have to buy another PSU. In the end the price is pretty close and it would save you money in the long run. If you don't plan on overclocking at all, and figure you never will venture down that road then the PSU that the guys recommended will be totally fine.