GTX 670 will not be bottnecked by a PCI-e 1.0 16x, PCI-e 2.0 8x, or PCI-e 3.0 4x slot. Anything less then those it will be (e.g. PCI-e 1.0 8x or 4x, or PCI-e 2.0 4x). If you start to SLI that is a different story, that is when those slots gets saturated fast.
No it won't be throttled since you are on a PCI-e 2.0 16x slot.
If you hate fan noise get the GTX 670 Asus Direct CU II Top card, according to reviews its one of the quietest high performance cards available, the only one I've seen for sale yet is on NCIX. I'm personally waiting for the 4GB version of this card myself.
The ASUS DC II cooler is a revelation on the GTX 670, it provides incredibly low noise levels. In both idle and load the card emits almost no noise, it is actually quieter under full load than most other cards on the market are at idle. Not only noise is low but temperatures are great, too. With only 74°C under load the card is quieter than most other GTX 670 cards tested today.