CPU/GPU/MOBO bottlenecks. Upgrading PC.

Hello people

I want to start by saying that i'm pretty new to modifying computers so please forgive any noobish mistakes.

Right now im on a computer with the following specs (Ask if you need more information)

Motherboard: Gigabyte GA-P55A-UD3 with a LGA1156 cpu spot and a marvell sata3 controller, which I suppose is bottlenecking.

GPU: ATI Radeon HD 4870 x2

CPU: DualCore Intel Core i5-660, 3366 MHz

Ram: 16gb DDR3 1600 mhz.

I've had thoughts about upgrading my graphics card since it's pretty outdated, but I'm affraid that my CPU will bottleneck the GPU upgrade.
Does anyone have any experience overclocking the i5-660 with stock cooling?

Since im not 100% confident with modding a PC I'd prefer not to change motherboard..

Sorry if the thread is a bit confusing, it has to do with English not beeing my first language. Please tell me if I need to add anything to this post

Thanks for reading