[Tristan Cook]"I was wondering how important a graphics card is for previews and rendering."
VERY important. Without a good graphics card that enables the Mercury engine, editing many native formats won't be as smooth as you'd like. Without the Mercury Engine, playback can stutter...the machine's responsiveness won't be as snappy as you might be used to.
[Tristan Cook]" but will the RAM compensate for the lack of graphics card and is a graphics card faster?"
It will not compensate. Really, for CS6...any version of Adobe Production apps...you need both RAM and a good graphics card. Not either/or. Both. And you need a specific graphics card to enable the Mercury engine.
[Tristan Cook]"Is it worth paying a few hundred pounds more for a dedicated card"
Yes, it is. One that is rated to enable the engine.