BP, you could happily run a Radeon 4850 with 2GB of system ram in Vista, it wont make a difference to the actual running of the card.
Its the software (ie games) which will make use of the system ram, and, most games still only use 2GB ram max, which was a limit imposed by 32bit windows.
I believe some games are becoming large address aware, so that they can make use of larger ram amounts, but 6Gb would have been absolutely fine.
You would have been better served getting a graphics card with 1GB of onboard ram instead of 512mb, especially useful for games running in high resolutions with large textures, shaders etc.
Its not going to hurt having more system ram, and as long as your happy its fine, but I feel it was definitely unnecessary for Dell to imply that you need 8GB over 6GB purely for selecting a Radeon 4850.