Don't listen to these idiots. If you want room for expansion, get another rig. But this mobo you picked will allow you 3 cards. I own about 6 of them. That is a perfectly fine setup EXCEPT go 1gb or less ram and you can get a 40gb hdd from eBay used for like 5 bucks. You do not need 120gb. The PSU you are getting will allow you to power at least three 5850s overclocked pretty high.
And whoever told you 5850 is just "okay" for mining, they are stupid like most the people on this forum. That is a top 3 BEST CARD you can get for energy efficiency and hash rate. I own about 40 of them. Hate any other card besides 5970 and 7970.
And I would seriously not recommend gaming with the rig. Just let it mine and earn you money. Gaming is harder on GPUs than mining. But this is coming from a person who mines for a living, not just for fun.