If you play large games,
(BF2, BF2142, allegedly Crysis and some others) which can come near to 2BG usage, or if you do memory intensive things like video editing, or like to have 500 thousand windows open all at once, then yes, you should definitely upgrade. You could argue it's kind of overkill if you don't do that much memory intensive work. But DDR2 is cheap now, and 3.25 (displayed) is still 55% more than 2. Why not?
Just be aware that many motherboards won't drive 4 sticks of high performance (1066, etc) RAM to full spec. Sometimes you can get away with a voltage bump and looser timings, but there's still a good chance you may have to drop it a speed setting to get the system running stable. Running 1066 RAM at 800, for example... Tho, you have to admit it could be turned into a fine excuse to bump the FSB to 400: E or Q6600 @ 400 FSB = 3.2Ghz with 1:1 FSB:RAM ratio.
Scott