Even so, I think that upgrades from one years round of gfx cards to the next rarely gives much absolute performance increase nowadays, when considering similarly priced models, certainly not a dramatic increase to justify how expensive they are. As such, going from a nvidia GTX2XX (excluding 210 etc - 260 or above) to any new card (radeon 5xxx) seems like mostly a waste of time unless you've got a GTX260 192 and are going to 5970 or similar. Then you'd notice an increase in performance, but almost all today's games are still perfectly playable on med/high settings with the last generation of cards. I think it may take until the 5xxx refresh (5890 anyone?) or even the Fermi refresh to look spending some serious cash upgrading. Of course, that depends on the demands of the new games that come out too, and if you play them! If you're still playing source games only there's little benefit from an 8800GT to be fair, they're still very playable indeed. Changing to a larger monitor may create a need, but it also depends on how much cash you can afford to throw at keeping your rig up to date. Personally, I think the gfx market needs to move in a new, more productive direction, like the CPU market has moved to more cores instead of absolute speed.
