This may have been covered before now but looking at lap times over the past few years, the 2004 cars really do stand out compared to the others. For instance, in some tracks (i.e Silverstone) the 2004 cars are almost 4 seconds faster than the 2003 cars. As far as I know the regulations didn't change too much (rear wing elements were reduced for 2004) but still, the cars were so much faster than anything else that'd come before and are still a lot faster than 2008 cars.
In 2008, we're starting to catch up again to the 2004 cars. For instance the 2008 cars are faster than both the 2005 and 2003 cars on many circuits (almost always faster than the latter)...but still, the 2004 cars sit out on their own as the fastest racing cars ever created, and by some margin.
In terms of pace, it's roughly:
2004
2006
2005
2008
2003
2007
It obvious depends on the tack but it's something like that in general for most tracks.