Well, you should try to find some articles which compare 800mhz vs. 1066mhz. The general though however is: lower timings is better than higher speeds. It's all up to you and your budget. If can only afford DDR2 1066 with high timings it's better to buy DDR2 800 with very low timings. However if you can afford ddr2 1066 with the lowest timings, which shouldn't be a problem as ddr2 prices are low, go for it.
The only reason I can see why some would prefer 800mhz over 1066mhz, is because it is easier to run at a DRAM:FSB ratio of 1:1 with ddr2 800 than with 1066. This is mostly considered as the most stable configuration, but you shouldn't care too much about it as it is seriously overdone.