It does not slow your system down; many cards come with software's adjusted especially for the card. whether it's information only, programs only, overclock utilities and factory drivers which hold essential information regarding your card they all have impact on the cards stability/performance.
I personally have the original and the new drivers installed on my system, they both work correspondingly with no problems what so ever, plus I have my full information and utilities, which I installed with the original CD.
It does not slow my system performance at all, the only thing that could possibly slow the performance is installing more then one new drivers without deleting the previews one.
He can choose to install only the new drivers or original with the new drivers, it's up to him
From my experience, I prefer having the information, utilities and drivers, which came with the card's CD and at the same time 67.02 drivers. Both work very well together.
*UPDATE*
You can install new drivers over old ones with absolutely no problem, install as much as you want with out leaving fragments.
The special feature, which NVIDIA drivers have, is that they "take care" of your old drivers for you. Therefore, you do not need to delete the "old" drivers before installing new ones.
