Which is better ? 2X4 or Single 8

[watercooled]

Cost and the possibility to upgrade in future are about the only major considerations really.

In some cases, memory controllers may struggle to hit higher clock speeds with all slots populated due to the additional loading, but that's generally only a concern if you're using very high speed memory and/or overclocking.

[Agent]

It's less of an issue now the MC is part of the CPU. It caused much more hassle when it was part of the northbridge, but it's something to be aware of.

[Extinc]

2 x 4gb is faster while running in dual channel if they are the exact same but if you aren't running in dual channel or the memory isn't the same than the single stick is faster.

[watercooled]

If the memory if different speeds, the memory controller will generally fall back to the highest common speed. If they're different sizes, depending on the controller, it will still be run dual channel where possible, and any excess will run single.
http://www.intel.com/support/motherb.../CS-011965.htm

Aside from the potential loading issues I mentioned (which as Agent pointed out, are fairly rare on desktops nowadays) using multiple sticks in single channel shouldn't be slower than a single stick.