Crossfire with 4x PCIe

[Kasm]

I know 2 8x are better, but how does 1 16x and 1 4x handle Crossfire? Is it even worth it or is it a waste?

I am trying to pick where I should spend extra money, and the 2 8x boards seem to cost more. Plus I am not sure if I will ever use crossfire. I haven't in the pass even though all my boards support it. My updating cycles seem to be two far apart to make buying a second card useful with all the new cards out.

So basically is the 2 8x vs. 16x and 4x enough of a handicap that it is worth paying the extra just in case you later decide to go crossfire?

[Bobnova]

Depends on on the cards, given that a 5870 looses 1-2% at 8x, i'd expect a 5770 to run full blast or very close to it on a 4x.
A 5870 on a 4x looses something like 10-15% i think. Still not that bad, really.

Wish i had bookmarked the article on 5870 PCIe speeds.

EDIT:
HA! http://www.techpowerup.com/reviews/AMD/HD_5870_PCI-Express_Scaling/25.html

4x looses 5% at the absolute worst for a 5870.
You'll be fine with 16x/4x.

[jason4207]

Is this PCIe1.1 or 2.0? Just asking b/c I'm not used to seeing a x16/x4 lock except on older P35 boards.

[Kasm]

Well, the board I was looking at was MSI P55-GD55
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130248

After taking a closer look, it looks like the 4x is just pcie gen1 slot. How does this effect things? The board says it supports crossfire. This isn't the only P55 board that has this set up.

[jason4207]

No they are both PCIe2.0. With P55 there are so many ways to do things, and I haven't checked all the new boards out yet. With socket 1156 there are 16 PCIe2.0 lanes that go straight into the CPU, and 8 PCIe2.0 lanes that go through the SB. With the 16 direct lanes into the CPU the mobo manufacturer can decide to keep them dedicated to 1 slot at x16 or have them split between 2 slots and go from x16/x0 to x8/x8 when 2 cards are inserted. Then on the SB they have to save some lanes for NIC, etc, and they can break the bandwidth into x1, x2, and x4 slot(s). Typically if you use the x4 slot then the x1 slots turn off.

Hmm..just looked up your board, and the Gen1 vs Gen2 thing is confusing...I'm not really sure anymore. Unless they mean Gen2 is direct into the CPU , and Gen1 is off the SB.

More reading, and I found this:

Lynnfield has enough PCI Express lanes for most graphics configs. What about expansion slots and peripherals, though? That's where the P55 Express PCH comes in. It's equipped with eight gen-two PCIe lanes—two more than the old ICH10R south bridge. However, these lanes only offer signaling rates up to 2.5GT/s, which is the same speed as gen-one PCI Express. That's unlikely to be a major impediment in mid-range systems, but it will limit the bandwidth available to high-end RAID cards, and the like.
http://techreport.com/articles.x/17513

Looks like that x4 slot may be bandwidth starved.

That board isn't ideal for Cross-fire, but it will work. I'd go for a board that does x8/x8 if you want that functionality.