The Asus nForce4 product range now starts with the A8N-SLI and goes up to the Deluxe and Premium editions, with the last being the most expensive variant. At CeBIT, competitor MSI already showcased a Pentium 4 based nForce4 motherboard that allows for SLI switching without touching the hardware. However, since that was a prototype only, Asus has a nice unique selling point right now.
Unlike virtually all P4 motherboards, Socket 939 Athlon 64 motherboards should easily be able to host a dual core processor, based on the Athlon 64 Toledo core targeted for 2H05. All you should need to do is install an updated BIOS version. From this point of view, the investment in a rather expensive motherboard is easier to justify.
Until today, turning SLI support on and off required the computer to be opened. Asus makes this step obsolete, but introduces the necessity of an additional reboot. The first restart will activate hardware SLI support, while the second is needed to turn on SLI driver support. You also have to buy the most expensive board in the product line to get that convenience.