Look at my specs... I'm perfectly happy and have been playing Crysis at 1920x1200 max settings. I have no need to upgrade.
If you upgrade to SB, you're going to notice the system overall is much faster on boot and loading applications. Other than that for games, you won't notice much except better benchmark numbers.
That's my $0.02
Edit: If you crunch, program and compile, you will notice a difference, but nothing related to a GPU bottleneck.