It also depends on that quality of the memory you're planning on getting. If you're looking at high-quality memory, you'll probably be safe overclocking it with your processor if you go that route. In that case, 533 DDR2 would be fine with your 1066 FSB as you could overclock both, or leave both at stock speeds and maintain the 1:1 ratio. If you're looking at cheaper memory but you want to overclock, you may want to either get the 800 speed DDR2 or change the CPU:RAM timing ratio so as to not blow your RAM. Of course, the cheap 800 DDR2 may be just as much as the higher quality 533 DDR2...I haven't price-checked those lately.
Its really important you match ram speed to mother board. When you read all the new egg feedback of memory problems or ram that does not work with mother boards it usually a poorly match ram speed and voltage to mobo bios.
some low end mobos need 1.8v memory to boot (p5b-vm)
some high end mobos need 1.8-1.9v memory to boot and flash for higher memory (striker extreme)
many mobos will boot right up with ddr800 low latencies 4-4--4-12 you can push it too 4-4-3-10 thats about the lowest latencies at ddr800 around 2.2v.
above 2.2v you run into alot of degradation of memory - such as the 680i eating 2.4v ddr1000+ - stay away from this stuff waste of money!
most higher end mobos with voltage settings the optimum memory is ddr 800 4-4-4-12-1 at 2.05-2.2v
when you see xyz memory with all these combos most are the same memory 1.8 5-5-5-15 runs 1.9 4-4-4-12 at 667 or 2.1v at 4-4-4-12 or 4-4-4-12 at ddr800.
For overall system performance, like running PCMark, the difference between running 533 at 1:1 and running 800 at odd ratios is like 3-4% max overall syatem performance gain. Yes DDR800 has more bandwidth than DDR533. But since DDR800 (1600 effective) is effectively running way higher then the CPU's FSB of 1066 then the memory will always be waiting for the CPU and you cant use its full potential.
In my specific case, I ran PCMark05 twice with same CPU OC, 9x300 for 2.7Ghz. 1st run I had the memory at 1:1 ratio DDR600 4-4-4-12. I scored like 6615marks. Next run I use a ratio to run my memory at its rated DDR800 4-4-4-12 and scored a 6650marks. Less than 1% increase going from DDR600 to DDR800 on PCMark05.
The only time you ever buy the overpriced DDR1000 is if you can run a FSB of 500. And even then these guys are only getting a few percentages better performance then my cheap DDR800 running at 700. Assuming we have the same CPU clocks. Raising the CPU speed is where you are going to get results that are noticeable. That and the video card.