Regarding the 8-pin/6-pin question. The X3 comes with 4 6pin and 2 8pin. Their original design had 6pin plugs on the PSU, and then you used the included 6pin -> 8pin cables if you needed an 8pin. Ultra revised their design so the PSU had 8pin jacks and then if you needed a 6pin, you would use the included 8pin -> 6pin cable. The second way is a better way to implement cabling, but it's not like there was a real problem with the first design. Now I can't find any info on whether this revision has happened, Ultra's website still shows pictures of the old design. Either way you will be fine.
Regarding the gt280 sli, you should be fine. We know the TDP for those cards is ~240W. While that isn't it's exact power consumption, it does give us a good idea of what it will be and it should be around 240W. I would feel comfortable with the Ultra or other 70A psus when paired with an OCed quad and gt280 sli, but more is almost always better so if you can afford it, an 80A psu would be a slightly safer bet. For example, this OP1000
is a great deal for the same price as the Ultra after rebates and it has 80A (it is an awfully big rebate and I hate rebates so I can understand if you passed that up for the Ultra). But honestly, we are just guessing until we get some actual power consumption numbers, hopefully someone else can offer an opinion.