PC1066 is faster than PC800. Will you notice the difference most of the time ? Probably not.
If the reason for adding additional memory above 256m is just for a general performance increase it doesn't make sense to me to use the slower PC800 memory. That is like taking one step forward and one step back and paying for the privilege of doing so.
On the other hand there are specific applications where the additional memory will speed things up considerably more than going to a slower memory speed will slow things down. The only way to rationallly determine this is to do some research on the specific application you want to use.
If the reason for adding additional memory above 256m is just for a general performance increase it doesn't make sense to me to use the slower PC800 memory. That is like taking one step forward and one step back and paying for the privilege of doing so.
On the other hand there are specific applications where the additional memory will speed things up considerably more than going to a slower memory speed will slow things down. The only way to rationallly determine this is to do some research on the specific application you want to use.