I don't get this whole 3dmark obsession either, especially as it doesn't even take into account many new 9.0c features that games like FEAR, Battlefront 2, Quake 4, etc all use.
I spent a little over $400 for my upgrade (only fools pay the $599 listed on Dell's site) and 2 days later (post upgrade) don't regret it a bit. F.E.A.R. showed the biggest difference for me, followed by Quake 4 and Battlefront 2, all new games I play a great deal. I noticed F.E.A.R. the most only because I was mid-way through it when I switched cards; with the 6800 Go Ultra I got stuttering and low frames at 1680x1050 (1920x1200 was unplayable for me), with the 7800 everything was silky smooth without a hitch in sight. After flashing to 450/1200 I was able to run 1920x1200 as well, with everything turned up except volume shadows. Battlefront 2 now runs much smoother, especially the space missions.
3dmark scores and older, non-graphic intensive games like WoW (I play WoW too and believe me, it isn't the video taxing the hardware) don't tell the whole story and that is all there is to it. You can try to make yourself feel better for spending the money or try to make yourself feel better for NOT spending it and argue on internet forums all day long, but facts are facts - the 7800 is a big jump for newer games.