Hi newcomer and welcome to the Tom's hardware forum.
The P6X58D Premium is a expensive and a little old, go with the new version ASUS P6X58D-E
that have a very similar performance that the P6X58D Premium and with the same features for a less price.
The Ggaibyte UD3R is a good mobo, more cheap that the other two, but with a big problem when u use Crossfire or SLI. What problem? don't support USB 3.0 whe u use multi GPUs, that is something that u want in the future.
The Rampage III is the queen X58 mobo, but is very expensive to, it's a puer OC mobo with exclusive features of the ROG edition. IMO for the OC that u want and the future Crossfire the P6X58D-E is more that enough.
Finally the best RAM price/performance that u can get for ur rig are this G.Skill PI series 1600
, but if u want something much better the way are this Mushkin Redline 1600