What about your DDR memory; How fast is it? Most new CPU's work best with DDR400 or even DDR2. DDR266 or 333 will probably work, but should lower the overall performance.
...or where you planning on getting new memory too?
(Some personal experience with graphic cards: I bought a radeon 9800 pro about 18 months ago. At that time it wasn't the best of the best, but close. At this time, I have to turn down some visual eyecandy in modern games. I doubt I'll be able to keep it going for a full 3 years... I don't think a Nvidia 6800 will do much better.)
(Oh, and Doom 3 runs fine here. I dread oblivion, though.)