What OS? In short, 1066 will not be any faster than 800 unless you are overclocking. But an OS such as Windows XP 32 bit cannot access more than 4 GB of memory. So, if you're going with Vista 64bit you can go ahead with the 8 GB. Otherwise, it doesn't matter.
4-6 GB is enough, 8 is needed if you run more than 2 VMs or run simultaneous users/server apps. 8 GB is not going to be put to the best of use without some serious sized applications, databases, video editing, or virtual machines. if you are playing games and overclocking a dual core, go 1066, otherwise any DDR2-800.