HD3850 + PCIe 1.1 4X = Limited Performance?

[Von Matrices]

Hello everyone. I am looking for some advice with my system. Today, my colleague at work offered to sell me his stock-clocked ATI HD 3850 for a ridiculously low price. Right now I have an HD 3870 crossfire setup on a DFI X48-T3R, and if I did buy his card, I would have to put it in the 3rd slot which is PCIe 1.1 x4. I know that the performance improvement of 3-way CrossfireX, even in optimal conditions, would be minor, but what I am more concerned about is whether the PCIe 1.1 x4 limited bandwidth would severely limit the card's performance. Disregarding price, would the extra card in Crossfire with the PCIe 1.1 x4 slot offer any performance benefits over my current setup in graphics-limited games? I would appreciate your help. Thanks!

 

[Sylvanas]

It will improve your fps by how much it is hard to tell, but a 3850 in a 1.1 4x slot is usually limited in the realm of 15-20% in worst case scenarios. If you OC, raising the PCI-E frequency (at your own risk) would help alleviate the bandwidth constriction on a 4x slot (and the other slots devices on the PCI-E bus for that matter)- I run my cards at 125mhz which is usually regarded as the max before you may encounter issues.

 

[Von Matrices]

Since the slot is connected to the southbridge instead of the northbridge, would the PCI express frequency affect that slot as well or only the slots served by the northbridge? I'm afriad of overclocking the southbridge and potentially causing corruption on the hard drives connected to the integrated storage controller. Even with stock PCI express frequency, I think I might buy it and try my luck. Just out of curiosity, would a 3870 offer any performance improvements over the 3850 considering the bandwidth limitations?

 

[Sylvanas]

Originally posted by: Von Matrices
Since the slot is connected to the southbridge instead of the northbridge, would the PCI express frequency affect that slot as well or only the slots served by the northbridge? I'm afriad of overclocking the southbridge and potentially causing corruption on the hard drives connected to the integrated storage controller. Even with stock PCI express frequency, I think I might buy it and try my luck. Just out of curiosity, would a 3870 offer any performance improvements over the 3850 considering the bandwidth limitations?

You make a good point. A 4x connection to the SB would be communicating through DMI as Lopri explains here . In this case a faster PCi-E frequency would probably have no affect. I would buy the 3850 if you think it is a real bargain price to add to your current 3870CF setup.

Just out of curiosity, would a 3870 offer any performance improvements over the 3850 considering the bandwidth limitations?

Probably (I say 'probably' since nobody has really tested such a thing but from various posts round the net this is the consensus) no performance benefit worthwhile, most 3850's can clock up to 3870 speeds anyway.