You misunderstand what SLI is.
SLI is a system whereby nvidia NFORCE4 SLI owners ONLY can insert two identical video cards into who identical PCI-E x16 slots in order so that both cards output to the SAME SINGLE OUTPUT. This nearly doubles GPU processing power and has absolutely nothing to do with how many displays the actual images are being sent to.
SLI only works on nVidia GeForce 6600GT, 6800nu, 6800GT, and 6800ultra. And it ONLY WORKS on motherboards equipped with TWO PCI-E x16 slots and the board MUST HAVE an NFORCE4 SLI CHIPSET. That's IT. It is not available in any other GPU cores, period. It is not available in another other chipset maker's chip (via). Your PCX 5300 having dual outputs for dual display has zilch to do with SLI. As an example, my SINGLE 6600GT PCI-E card has DUAL OUTPUTS, but I only have ONE CARD on a NON-SLI motherboard, so that clearly doesn't qualify as SLI.
If you don't know what SLI is, then you DEFINITELY don't need it.
And there are a ton of differences between the VIA and nVidia chipsets, but SLI is not primary among them, at least not as a reason for the average buyer to choose one over the other.
Just forget SLI and look at the other features of the chipsets you are choosing among.
My advice: futureproof yourself and buy a socket 939 NForce4 Ultra-based motherboard. NON-sli
.