1. Usually no. If your card is overheating for some reason (mainly due to broken fan), or if you overclock and want more performance, then yes.
2. Accelero Xtreme III is one the of top VGA coolers compatible for the GTX 670. Watercooling for if you are seriously thinking about overclocking. There's no need to spend more for an aftermarket VGA cooler if you are not overclocking, or just planning to overclock no more than 100 mhz.