Why choose 1066 ram? Get the ram that's on sale, which is usually 1333 or 1600. I chose 1333 ram recently when I upgraded to 16 gb to save money. So, the answer to your question is a qualified "yes", and it still depends on how much ram you're running. For some folks, 4 gb is plenty.
some techie's would say better more than faster. but being 1333 with cl9 is somewhat 1066 with cl7 latency? well the speeds are theoretical if u dont have memory intensive apps. and with superfetching of win7 the bigger ram the better. as it would cache frequently used apps.. i use 8gb ram, and i use up to 6gb(2.5gb prefetch)
The main thing to consider is what your CPU can address. There is no point buying super fast RAM if your core is too slow to make use of the extra Mhz. But likewise, it seems a shame to buy a decent core and not let it stretch its legs a bit.
There will be an equation somewhere that details the differing speeds of high performance RAM compared to high capacity. I would imagine in this case, since you are talking about doubling your capacity and only dropping your speed a little bit, the 8GB should be the faster.