Replacing a 4GB DDR2 667 with a DDR2 800 one won't give you a performance boost in gaming/apps.
But getting a 8GB DDR2 667 will give you a performance boost(not much though since many apps/games run fine with 4GB)
This is an irrelevant question. There are different answers for this depending on what you are looking to accomplish.
4 gig of memory is plenty enough for all but the most demanding users. Are you doing anything requires you to have more than 4 gig of memory? If so, but I stress you would have to be working with very large files and demanding applications to need more than 4 gig, then adding more memory will help. If you are not using over 4 gig at anytime, adding more memory won't do a single thing for you, it is a waste of money.
The speed of the memory is another matter. At stock processor speeds, the speed your memory is running at is fine. Faster memory will do nothing at all for you here either. If you are trying to overclock your processor, the faster memory will allow for better tuning and slightly better performance related to your overclocking. At Stock speeds, it simply does not matter.
So there you have it. When talking about memory --first important is simply that you have enough of it. Once you have enough, then the final tweak is the speed, if you are into overclocking. If not, changing memory speeds and trying to just overclock your memory is really pointless. It really is.
So taking all of that into consideration, now perhaps you can better decide what to do that will benefit your particular circumstances.
Personally, from just reading your first post, you should just leave it like it is, because if you spend the money, I think you are going to feel like you spent money for nothing after it is all said and done.
I agree with all them the extra 4gigs is not gonna do you any good even if you did have a 64bit OS. And as for the speed the only advantages you would have is in overclocking but most Intel boards dont overclock.