No Intel server boards will let you run SLI. They might have two slots X16 slots, and may even say they support SLI, but in actuality the drivers don't work for their server chipsets.
There is a very easy pin-mod (requires a small piece of electrical tape) that will take your E5310 to 2.0GHz (1333MHz FSB). Beyond that, you'll need clockgen and some luck. You will probably not see 2.4GHz with the E5310s; get the E5320s if you want 2.4GHz.
The models above the E5320 come with a default bus speed of 1333MHz, so the easy pin-mod overclock won't work.
Intel will release LGA775 quad-core Xeons later this year for single-socket boards. They might be cheaper than the Core 2 Quad desktop CPUs.
Unfortunately, Intel doesn't have any dual-socket enthusiast systems on their roadmap. AMD does (4x4), but their quad-core CPUs won't be out until Q2 (Opterons) and Q3 (FX).
That is a shame, because these Clovertowns have a lot of headroom for overclocking; the platform just doesn't support it.