386_junkie wrote:Ok, I didn't realise the system itself may not be top-of-the-line regards to PCI-e.
You're missing the point.
What's the most powerful GPU today? Say it's the GTX 1080 Ti? Well, if you want performance on a level beyond what a single 1080 Ti can provide, you buy two (maybe even three) and put them in a SLI setup. This gives you the absolute best performance, assuming the rest of the system will not bottleneck them.
As soon as you leave the bleeding edge - SLI loses its value. For example, suppose a GTX 1080 is about twice the performance of a GTX 1060? Well, in most practical cases (there may exist some corner situations), it's better to just get a single GTX 1080 than a pair of GTX 1060 in SLI.
Unless it's like my other example - you bought a GTX 1060 because you didn't want to pay for a 1080, then 1-2 years later you feel like upgrading performance, and can get another 1060 cheaper than an 1080 (obviously) and don't want to bother with swapping out and selling your existing card.
That's it. Setting up SLI for an older platform has little practical value - it's only for the fun of experiencing something different.
386_junkie wrote:What i'm trying to ascertain is... with a Socket 939 system, with the fastest CPU available to it, where in the range of PCI-e cards would suit best? ... would the system support more a mediocre PCI-e card, or would it be able to support a high end PCI-e card?
You are talking about a 12-year old platform, whether the fastest CPU is an outdated-arch dual-core, yes? Obviously the current top-of-the-line cards will be severely bottlenecked. Heck, you are talking about PCIe 1.0, whereas today's platforms are PCIe 3.0 (4 times faster bus).
So if you really want SLI for the fun of it, something retro, almost period-correct and inexpensive is perhaps the 9800GT that BinaryDemon suggested, or one of the Quadro setups suggested by The Serpent Rider.
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