It's up to you if you want to overclock or not. If you plan to overclock, not only do you need a k-series CPU and compatible motherboard, but you need a proper CPU cooler and ventilation in your case, all which can add a bit of cost.
The graphics card can be overclocked on any motherboard, but the same applies to the GPU in regards to cooling. Overclocking will produce more heat which you will need to push out of your case.
Overclocking is somewhat of a linear performance improvement. If you have a 3Ghz processor and overclock it to 4Ghz, you can expect to get pretty close to 20-30% more performance in benchmarks that will utilize the extra CPU performance. Most applications aren't coded to use 100% of the CPU at all times, so you won't see nearly as much of a real world benefit however.