All mobo's that support S939 CPUs now have BIOS updates that fully support the X2's as well, so that's no reason of concern...
We also generally do not recommend SLI as an upgrade option. The main benefit of multi-GPU technology these days is to increase the maximum performance beyond the fastest single card on the market. ATI and NVIDIA have kept upgrade cycles fairly consistent over the past few years. It makes sense to spend money on a card that will bring increased performance and more features to a system when a new generation of GPU comes out rather than augmenting an aging card with another of the same type.
Coming from this very same AnandTech site...
EDIT: Some alternatives...
MSI K8N Neo4 Platinum (nForce4 Ultra) - $ 123 Great board, strong OCing, lots of features...
EPoX EP-9NPA+Ultra (nForce4 Ultra) - $ 105 Among the best and cheapest nForce4 Ultra mobo's available today, a very strong performer and great OCer...
EPoX EP-9NPAJ SLI (nForce4 SLI) - $ 120 AIS Very good SLI mobo, excellent overclocking as well, and a low price too...
EPoX EP-9NPA+ SLI (nForce4 SLI) - $ 137 Its bigger brother... Better, but it costs a tad more as well...
Plenty of reviews on those boards...