upgrading to an i7 isn't necessary if you are only going to be using it mainly for gaming, an i5 will be sufficient. However, I hope you realize that you motherboard is a Socket 1155, so you could only upgrade to an 2nd or 3rd generation i5/i7 procesor. Also, there is no need in getting a "k" series cpu, your motherboard does not support overclocking.
If you are wanting to get a 4th generation cpu, then you will need to upgrade your motherboard as well, which probably means a new OS disk/key also.
Just save enough to get a Z87. It's more "future proof" because even if you don't want to overclock now, you may want to in the future, and you'll have the option. Plus, Z87 mobos have all the best features and are usually are not cheaply made like the lesser boards can be.
There's really no "con" to getting the Z except for paying a little extra, but really, the motherboard is the single most important component of any build so do skimp on it!