Pushing the timings from CL5 to CL4 is by no means a sure thing.
Doing so would essentially overclock your RAM and it may become unstable.
From the two choices you list, I would probably spend the extra $8 for the DDR2 1066.
It will be slightly faster and is priced within spitting distance of the DDR2 800.
I think a better choice would be between This
($85.99) and This
($75.99 after $25 MIR) set of CL4 DDR2 800 and your above listed DDR2 1066.
They will all give you similar performance.
Personally, I would go for one of the less expensive DDR2 800 kits, set the FSB at 400Mhz and drop the multiplier until the overclock is stable.
This will let you run your RAM at a 1:1 ratio with the FSB, use your motherboards 400Mhz bootstrap and give you a good overclock in the process.