at this point in time there is little performance difference between the 8x and 16x bandwidth except for most high end cards now use the 16x bandwidth. In the future (hopefully near) games will utelize the full potential of the 16x pathway and AGP and other 8x cards will be obselete. Because most people in the future operating at the 8x frequency will be people with an SLI board, it won't matter beacsue 2 cards is almost always better than 1 (e.g. 2x 7600gt>1x 7900gtx even on a "normal" SLI board) I would go for that board because it is still around the same price as any DFI sli board and you'll be future proof, probably for a long time. Like i said, DFI is really expensive, but you definetly get what you pay for, especially if you plan on overclocking. In conclusion, no, its not worth the extra $40...right now. If youre on a tight budget, go for the basic ASUS a8n-sli as opposed to the deluxe, premium, or a8n-32 (they all pretty much have identical features) and not abit because I've heard a lot of bad reviews. Like I said too, If you plan on overclocking... GO DFI.
Good Luck
PS heat pipes on the MoBo are overrated if you have a good cpu and gpu heatsink (thermalright xp-90c and zalman zf900)
PPS whats your current graphics card?