A single 8800GTX paired with an E6600 or better CPU exceeds 12k in 3DMark06... those cards are just insanely powerful. And yes, a pair of those is too much right now
Best overclocker: Intel P965 Express. Low budget boards with a single PCI-E x16 slot (Gigabyte S3, Asus P5B); more expensive ones with two (Asus P5B Deluxe). Those with two support Crossfire (bandwidth at x16, x4 only though), and can handle SLI with modded drivers (not sure if there are any newer versions of those modded drivers)
Best for Crossfire: Intel 975X. Good overclocker, Crossfire at x8, x8. SLI with modded drivers. See comment above about drivers. A good one is the Asus P5W DH Deluxe
SLI: 680i. SLI at x16, x16. Best is the Asus Striker Extreme (but that one is $400). I'd wait a bit to see how these really pan out.