At the start of July I purchased a radeon 4870, however I would now like to upgrade. Buying a second radeon 4870 would be the easiest option, however I have a P45 chipset and various reviews show that crossfire performance suffers at high resolution/with anti-aliasing due to the 8x/8x crossfire as opposed to full 16x/16x.
As such I was thinking of selling the 4870 and buying a 4870X2 that will not be affected by the 8x bandwidth limitation. However I am unsure what the market is like for (admittedly relatively new) hardware and not sure how much I could get for the card/where to sell, ebay? I know new cards are still around 185 pounds.
Technically, the 4870X2 is affected by it, as in that case, it likewise has only 16 lanes of PCI-express bandwidth split between the two GPUs. It's just that it can re-distribute them as necessary. And if memory serves, P45 is PCI-express 2.0, which means you'd be getting bandwidth equivalent to x16 PCI-e 1.0 with just 8 lanes.
Even with reduced bandwidth, if such a thing did happen, it wouldn't really affect performance. So really, I think that selling your card and going for a single 4870X2 rather than a pair of 4870s is perhaps a somewhat silly idea that will just cost you a bunch more for what will be, at best, just a hair more performance. Oh, and perhaps 1GB of memory per card, but it doesn't sound like that's terribly important to you.
A review on tweaktown shows two crossfired 4870's suffering due to the limited bandwidth relative to the X2 - I believe the issue is do to communication between the two cards. Whilst I agree with you, in theory because in the X2 16 lanes are still shared between the two GPUs it shouldn't perform any better however that does not seem to be the case. This is more evident at higher resolutions/more bandwidth intensive scenarios e.g. crysis. I have a 24 inch monitor and res 1920x1200, but with a crossfire setup or x2 i would likely be running with AA on etc. where it may become pronounced. I have seen a forum post by a user comparing and showing little difference but there is no AA used there etc.
If I could sell my current 4870 for a reasonable price then an x2 would not be much more than buying a new 4870 to crossfire and as you mention, would have the added benefit of having 1gb memory which may or may not help if I have high AA on at 1920x1200. Obviously if I cannot sell the card for much I would, as you say, be better off going for a second 4870 for crossfire. Thanks - Boffinboy
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