Well, large sticks cost more, that's all there is to it. Make sure your board supports those sticks as well(some do not like 8 gig sticks).
If you are running windows, make sure to get Windows 7 Professional and not Home Premium. Home Premium only supports 16gigs of memory.
If you hold out for Intel's next big platform. It may well have 8 memory slots since it is a quad channel system. Current boards I see for that are at 4 slots, but with some luck maybe 8 slots are in the future.
What are you doing that is soo memory intense?