I think you'll be happy with the upgrade, the 6800 GT got me through HL2, Doom 3, and FEAR at medium/high settings with smooth framerates. Heck I'd probably still be using the card if I hadn't upgraded to PCI-E.
Also, I've used a 6600 GT in my system (during RMAs and such) and the 6800 GT
noticably out performs that card, so going from a 6200 to a 6800 GT, yeah I think you'll like it

.