Maybe Dumb Question

1.) Nope, there are no boards which can hold different processors together. It wouldn't work anyway, the Phenom II X4 and Core i7 work completely differently. There are dual CPU boards, although normally you can only use AMD Opterons for AMD motherboards and Intel Xeons for Intel motherboards, the only desktop CPU you can use in a dual socket motherboard would be the Intel Core 2 Extreme Q9775. Dual socket motherboards are designed to be used as workstations or servers, where incredible amounts of CPU power is needed or computer stability.

2.) N/A, Q1's answer.

3.) The CPU will be running together, however, it won't necessarily be running at super fast speeds - for example, you can have dual Xeon E5540s running at 2.4GHz. They are both quad core CPUs which support HyperThreading, and thus also have four virtual cores. Thus, two Xeon E5540s have eight physical cores and virtual cores, 16 in total. However, they are running at 2.4GHz, and programs which aren't designed for more than say four cores won't really take advantage of the dual Xeons, and a Core i5 750 would be faster for applications which use less than four cores for example, because of the higher clock speed.

4.) No, unless you actually are building a professional workstation or plan to build a server. Dual socket CPUs tend to be quite a bit more expensive than their single socket counterparts. For example, the Core i7 930 2.8GHz costs approx. $294 USD. However, the Intel Xeon X5560 2.8GHz, the Xeon equivalent IIRC, costs $1172, around four times more expensive. Single CPUs are more than enough for everyday's desktop purposes.