ATI V5200 - need 8x or 16x PCI-e lanes?

[noetus]

I've got an ATI V5200 video card (the 256 bit model) and I'm thinking of upgrading my rig with an Asus P5W64 WS Professional motherboard. Since I'll be using a second videocard for HDTV and also a hardware RAID card on one of the PCI-e slots, the Asus will be configured as 8x 8x 4x 8x on the PCI-e lanes. So the V5200 will have to be run on 8 lanes, not 16.

I am using two 24" monitors running at 93Hz refresh, 1920x1200, 32-bit color.

The question is, will 8 lanes of bandwidth be enough for this card to run at its full potential with these two monitors?

[noetus]

Well, from searching around I've partially answered my own question.

I found an article on Tom's Hardware that tests different PCI express bandwidths for two high-end cards - more powerful than the V5200.

The conclusions are interesting and not entirely conclusive. It looks like that for the V5200, considering that it is lower-powered than the two cards on test (though not by a large amount for pro graphics work), it probably will not often matter that it is on the x8 bus, except possibly for certain high-end graphics pro tasks (and probably wouldn't affect gaming at all, though this is not a gaming card).

Note that the card's theoretical bandwidth is approx 256/8 (memory bandwidth in bytes) x 2 (memory is DDR3 so transfers at both clock edges) x 1380 (memory bus in MHz) / 1024 = ~ 86 GB/s. The limiting factor in deciding if an x8 or x16 bus is needed is not theoretic bandwidth (the card's bandwidth exceeds the lanes' bandwidth by a large factor -the x8 is ~5GB/s duplex, the x16 is ~ 10GB/s duplex ) but (a) what applications the card will be used for, and (b) other bottlenecks, eg on motherboard or in CPU.

This raises another question, though. The V5200 card comes in two flavors, one has 128 bit memory and the other has 256 bit memory. Give that the bandwidth of both these cards exceeds the available bandwidth on even 16 lanes of PCI express, what possible real-world advantage does the 256 bit version have?