Ok so here is all i know about the sandybridge stuff:
Starting at the Core i3, there are NO onboard graphics at all and it is compatible with both the H67 and P67 Mobo.
The Core i5 2400 and 2400k, only difference being the k denotes unlocked, both have onboard graphics, but it is only usable with H67 mobos. the P67 board does not support the onboard graphics capabilities of the new CPU.
The Core i7 2500 and 2500k, same difference as the i5, are exactly the same, they have the same overclocking capabilities and all, no need to dwell on it.
Now here's the new part: SandyBridge actually has another division of Mobos that aren't in full production yet that are built more for gaming that actually do support the onboard capabilities and allows the CPU to be stretched to its absolute limit safely. It's the SandyBridge equivalent to the X58, its called the Z58. this is going to be the top of the top of the top for SandyBridge, but, like i said, these aren't even in full production yet.
As of now, for gaming, you should get an i7 2500k and a P67 mobo. For a HTPC, an i5 2400 with the H67 mobo, reason I say the 2400 and not the k is because overclocking isn't exactly necessary. This is what i have so far on my research of the CPU and Mobo.