You'd see a noticeable difference in performance upgrading from a GeForce 6800 to a Quadro FX 4000. The biggest differences are in viewport performance and particle effects. Granted, the 6800 is the least powerful card supported so any upgrade will be noticeable.
If you're looking to upgrade I'd go with a newer GeForce (8000 series or newer). The Quadros are phenomenal cards but if you don't have other software that requires one save the money.