For emails and surfing, that much processor power will last you forever, even my old Athlon XP worked just fine. As for Photoshop and DVD conversion, that's asking for quite abit more power. As long as you're not needing DVD converted to Xvid in a hurry, it should be more than enough to do the work. You're only looking at about 20-30% encoding speed difference compared to top tier Core2Duo processors, not that a big deal in actual application. Quad core will do even better for DVD conversion by the way.
As for gaming, that's heavily dependent on your Gfx card on the higher resolution (who games at 800x600 anymore?) With your nVidia 7900GS and 4GB ram, I think you're good to go for at least a couple of years. My spec is comparable to yours, and I'm very happy with the result for my use. Your 7900 is one of the top DirectX9 card for a while and I would wait at least until nVidia 9xxx comes out before getting a new card. I play Oblivion, SupCommander at high res and it works out just fine. Even with Crysis I had to tune down the res but these ex-top DX9 cards are still doing a good job on gaming.
Also, E4500 can be pushed to 3.3Ghz without much trouble if your board is capable. That'll only get you maybe 5 fps more in games, so it's not really worth much. I got my E4500 up to 3.5Ghz but it was running pretty hot. It should be good to go for couple of years until DDR3 prices drop and Nehalem comes out.