I have an Airport Extreme base station and have 2 iMacs, 1 Windows laptop, 2 iPads, 1 iPhone, XBOX 360, Canon printer and Samsung TV all connected at 802.11n and have no real issues with slow downs..
The router is in the basement (a bad place I know, just haven't bothered moving it yet) and my iMac sitting on the 1st floor easily gets about 5MB/s download speeds. My daughter's iMac on the 2nd floor gets about 3.5MB/s and none of her streaming (videos from websites) is delayed/stuttering or anything.
I use the Samsung TV for streaming videos from Netflix and a local NAS and it also works great and the iPads and iPhone work without too many glitches..
One thing to keep in mind is that if you start mixing your devices at different 802.11 speeds, it'll cause a overall reduction of performance..
But as pendlewitch said, a dual-band router is probably not a bad thing to look into..
Also you would probably get overall better speeds if you just upgrade to a N router and you really should retire anything that's still stuck at G. So when you do go looking for the N router, having it be dual-band is a bonus..