Look at it this way- a poorly desinged converter will benefit from a stable clock source. That is one of the places that many converters lack. Any of the RME converters will sound better with a good clock, but it won't be a massive difference.
What would be a more important question is "how does my digital system integrate?" I find that a master clock makes everything easier to integrate when you have large setups (ie console, computer, converters, backup recorder, etc...). Just tell everything to sync to the external source and you'll be set. If you just have one set of converters going into a computer, you probably can survive just fine without.
As for preamps- there are tons of choices out there. My personal flavor of the month that I've really been digging is the new A-Designs Pacifica. It is an absolute first rate preamp.