Yeah, that kind of multi-tasking definitely isn't cpu-intensive multi-tasking. I suppose it's time for that term to be defined in different ways. For doing those tasks, RAM is your friend moreso than CPU speed/cores.
And if photoshop is important to you, I recommend testing out an LCD before replacing your CRT. You may or may not like the difference.
For that resolution (either 16x12 or 16x10) CPU speed isn't going to matter much with graphic-intensive games. You didn't mention what games you enjoy playing now and hope to play in the future. For example, take an FPS game like
Quake4. At that resolution, a C2D at 4ghz doesn't really improve gameplay at all over a C2D at stock 2.4ghz. That's almost double the CPU speed with really no benefit for gaming. RTS games tend to favor CPU speed moreso than FPSs. But, at that resolution, even
Company of Heroes benefits only slightly from that extra 1.6ghz in speed.
I guess my long-winded point is that if you're going to spend $250 on a CPU, it might as well be a quad, even if it's slower and has less overclockability than a 6850/6750 because, for games at your resolution, CPU speed matters little. In the future, if you find yourself rendering graphics or encoding video at the same time you play a game, that quad will come in very useful though, while the C2D will struggle with 2 dual-core apps being used simultaneously.
However, based on all this info, if you decided to drop down to a much cheaper C2D (since 2.4ghz would give roughly the same gaming performance as 4ghz) and put more money into your GFX card (where gaming performance at your resolution is really determined), that wouldn't be a bad choice at all either.