I went from a 7600GT (oc'd) to a 4850. A world of difference is an understatement. My main game is WoW and although my 7600GT can handle that well, the 4850 can handle everything max'd out at my resolution (which is 1440x900). It looks sharper, crisper, and the frame rate pegs at 60FPS everywhere.
But I've also gotten into other games lately. I finished Bioshock recently, and started playing Mass Effect and Hellgate London just last week. Bioshock is probably my favorite game I've played in the last 2-3 years and I "suffered" through it on my 7600GT. At the time it didn't feel like suffering but after playing the first 10-15 minutes today on my new card the difference is amazing. The game is absolutely stunning on the 4850 compared to a 7600GT.
I went from playing Mass Effect on about the lowest possible settings to everything max'd out at my resolution and I know the 4850 could handle more. I would get really bad visual lag with my 7600GT in the middle of the Citadel (basically the political seat of power for the galactic alliance) as there are huge buildings, pools, water spouts, walkways, and people. My poor little 7600GT just couldn't keep up there and I'd get single digit frames/sec even at the lowest settings. With the 4850 its not even breaking a sweat there.
And while Hellgate London isn't pushing the envelope graphically, it can bring a lower card to its knees on multiplayer in certain areas. I've gone from medium DX9 settings with no shadows and lower textures to max'd settings in DX10 and the game is silky smooth now.
So while you might not see a big improvement if you're playing older games, a 4850 (or 8800GT for that matter) will let you see everything you've been missing out on and let you get "in the game" for today's games.
Was I wowed? Yes, and then some.