Honestly .2ghz isn't very much, and for a $100 extra i don't really see the justification. That laptop would be able to handle stuff pretty well, if im not mistaken the 8600m gt is pretty much the same as the desktop version 8600gt which can handle games alright. on that resolution it would probably not do to bad, but i don't think you could expect wonders or highest settings.
oh and to the audio i don't know how dell does they're sound but that doesn't sound like much of a hardware improvement, but for $25 its up to you.