Theres no way an i7 Sandy would bottleneck a GTX 670.. The PCIe 2.0 instead of 3.0, reduces the bandwidth, but theres no visible difference.
About the drivers, you will be fine to update your system to the latest NVIDIA drivers before hand, then swap out your current card, with the 670. Don't bother with Drivers from the CD, they are most often out of date.
All you have to do, is remove the power plugs from the current card, and unplug any display cables. Slide the latch on the motherboard to unlock it, remove any bracket screws that could be holding the card in, then just gently pull it out.
Btw, be sure to not remove any stickers from the card's PCB, or dispose the box. I had to RMA my 670 FTW, and having those things still, eased the process.