Well I'm planning on getting a MSI 790FX-GD70 motherboard with AMD phenom 955. The board has 4 pcie slots, on there sites and there manual it says it runs at 16/0/16/0 or 8/8/8/8. Now I would like to ask, can i run 16/0/8/8? I'm assuming I can but, I'll ask anyway so I'm sure.
That would seem to indicate that you have two 16x lanes that are each shared by a separate pair of slots. Which would make your setup logical, as long as the motherboard's own logic can recognize it, which is not a guarantee. I would seriously e-mail the manufacturer with that one.
It probably doesn't matter anyway -- what cards are you planning on using? Nothing except the highest-end cards on the market today can even saturate an x8 slot. So unless you're planning on something like triple crossfire with 5870s, or triple SLI with GTX 295s, it shouldn't affect performance anyway.
Also, it's kind of irrelevant because once you get beyond two cards, adding a third card only produces minimal gains anyway (we're talking like 10-20%). So you would mostly be wasting your money ... and if you were using any card that can fill an entire x8 slot, you'd be wasting a shameful amount of money.
Basically, stick with two cards unless you're one of a very few people who has a specific reason to use more, or if you're one of those people who enjoys making your machine into a science project for the fun of it.
Well I was planning on getting an ATI 5850 in the first slot and leave the second slot empty so the 5850 has 16x pcie lanes, an usb 3.0 and sata 6gb/s card in the third slot and meaybe an raid card in forth slot.
And ye I', probably going to do crossfire in future with the 5850 so there's no really difference between 16x and 8x for a 5850 right?
For a 5850 I think that's right, you should still be OK at 8x/8x. It's mostly the high-end dual GPU cards you have to worry about saturating an 8x slot, though for some reason I THINK I remember reading that with the 5870 you lose something like 4% performance. So no, not really a big deal.
Honestly, though, I would skip that motherboard and just get one that has USB 3.0 and SATA 6gb built in, so you don't have to mess with controller cards or anything like that. Just an added expense and more things that can go wrong, if you ask me. Along the same lines, I'm not sure you need a RAID controller card either, or RAID at all for that matter.
Anyway, for what you're talking about, you might consider this board by Gigabyte instead:
It's only $15 more, and you eliminate the expense of a USB 3.0 controller, so you'll probably actually save money. Also, it operates at 16x/16x if you've got nothing in the third slot, so you can put whatever you want in crossfire and not worry.
If saving money is an issue, there's this one with only two PCIe slots (16x/8x) for $60 less...