Originally Posted by swarfega
Pretty much. DDR 800 is the top JEDEC DDR2 speed. 1066RAM just has chips that are rated to overclock. If you run your RAM at 800, it should be rock solid. Running at 1066 is an overclock (even if they're sold as 1066) and usually requires a boost in voltage to attain. With the boost in voltage comes an exponential increase in heat, causing instability unless you address the heat. That and some mobo voltage regulators may 'burn out' at that increased voltage. The 800 at 4-4-4-12-2t should be just as fast as 1066 at 5-5-5-15-2t.
And 4 sticks are always more taxing on the memory subsystem than 2 sticks. Adding 2 sticks will almost always lower the Vista WEI score for RAM, since that score is based on throughput and Vista has to access more RAM to test it. Average throughput will usually be lower the longer the test is run.