Well... SIGNIFICANT difference, no. The thing you want to do is get a good graphics card. You can see on my sig what I got. I ran Unreal Tournament at 1600x1200x32 with everything turned to max, loaded the 2nd Space Beacon map (it's so huge that 16 players can take a lot of time to find each other) and put in 16 contestants in a 4-team game and was getting an average of 60 fps. At no time was there any kind of lag, unless the OS flaked out.
At the time, I didn't have a useful CPU-utilization meter to tell if both CPUs were being used or not. I suspect they were, as I couldn't get this particular game config to work nearly so well on a T-bird 1300 w/ TNT2U, but I haven't retested yet.
No dually system will overclock the FSB very much. Maybe if you get really lucky. I got it up to 150, but it wouldn't always boot up. I was having some problems at 144, and have dropped everything back to spec for an extensive testing period to find out just where stability lies. I would not be surprised if 144 was the best I could do. Wish I could use multipliers, I'm sure these CPUs could go a lot higher.