My current motherboard supports up to ddr2 4200 without dual channel. I have a Pentium 4 at 800fsb, and will be upgrading to a Core 2 Duo or Quad with 1066fsb (if I keep my current board). I play games, but I don't need to have all the bells and whistles. How big of a difference in performance is there between dual channel ddr2 800 and ddr2 533 with my P4? With a Core 2?
I am assuming your Pentium 4 is 800MT/s (ie 200 fsb), so both ddr2 800 and ddr2 533 will have to be downclocked to run at a 1:1 ratio (the importance of this is arguable though), ddr2 800 runs at 400 fsb so without doing anything really but plugging in the ram you'll be good to go at a 1:2 ratio. As for ddr2 533, it runs at 266fsb, so youre looking at a 4:5 ratio.
At these minor fsb increases I would say timings are more important. So my suggestion would be, for the P4, either get ddr2 533 and downclock it to 200fsb and tighten the timings. Or if you will be upgrading to a Core 2 (1066/4 = 266 fsb) soon get ddr2 800 and either run at it at 1:2 ratio for now or downclock all the way to 200fsb and again tighten the timings until you get your Core 2.
When you do get the Core 2, overclock it to 333fsb and run the ddr2 800 memory at 1:1 ratio (speeds of ddr2 667), with slightly tighter than factory timings.
In conclusion, i would say, no, there is not that much difference between ddr2 533 and ddr2 800 with your P4, but when you get a Core2, definitely go with ddr2 800.
Actually, I think DDR2-800 will yield significantly better results for the Core2Duo.
The reason is that it will allow the Core2Duo to be overclocked which it does very well. There is almost no reason to not significantly OC the lower end Core2Duos to a much higher speed. This can be done without increasing voltage or spending much on after market coolers.