As for speculating: I'd rather do that with silver (A better deal imho as long as one oz costs less than a card
)
Yeah, silver is still my main investment but Bitcoins sure are eating into it nowadays.
Anyways, I think a MicroATX build rocking 2 6950s would be pretty wicked, here's some parts I think are great for the price:
Case - NZXT Vulcan
http://www.newegg.com/Product/Product.aspx?Item=N82E16811146065Case is a bit flimsy so don't manhandle it, other than that it can rock 2 full length cards in the case and has great ventilation and features (for $70 it can't be beat).
Motherboard - GA-A75M-S2V
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128516Seems like a solid motherboard (I've always had good luck with Gigabyte, great quality for the price). The only downside is only 2 RAM slots and there's a good possibility the bottom PCIex2.0 slot won't work with this case, although it would be nice to have that extra room between your cards to draw in the air. If this board won't work in the case I would go with this Biostar that will definitely work (you'll have to check into this for yourself):
http://www.newegg.com/Product/Product.aspx?Item=N82E16813138333PSU - OCZ Modstream 700W
http://www.newegg.com/Product/Product.aspx?Item=N82E16817341018Excellent semi-modular power supply that has shorter cables designed for microATX cases. Have had this model for 5 years and works great, might want to look at lower or higher wattage depending on the cards you plan on putting in your case.
RAM - Kingston HyperX 8GB (2X4GB)
http://www.newegg.com/Product/Product.aspx?Item=N82E16820104266Great RAM at a great price, you could put just 4GB in your computer but for how dirt cheap 8GB is now why not? You will probably never use more than 6GB, even under extreme circumstances though. Up to you, no big deal.
Overall I think this would make a great build not only for Bitcoin mining but for everyday use, all at a price that shouldn't break the bank and good quality components that will last you a long time. The CPU and video card will be the most expensive components though, that's for sure. I would look into 5850s, 5870s, and 6950s for this build since the case will fit them (whatever you can afford).