Hi newcomer and welcome to the Tom's hardware forum.
1- C2Q9650 is old socket so IMO isn't a good option.
2- i5-661 IMO is a little expensive, go with the i5-750 that is a better option.
3- It's only a dual core and can't give u the same performance of the i5 or c2q, the i3 is a good options but for laptops.
Best CPU you mentioned is the C2Q9650. The other two are newer tech but both dual cores.
What mobo depends on your budget, features you want (like good for overclocking, much pcie slots,...)
The chipset required seriously limits the choice of mobo's. The C2Q runs on LGA775, with most popular chipsets the Intel X48 (performance) and Intel P45 (mainstream, runs cooler). nVidia also has some chipsets like the nForce 600 and 700 series (check their site for more info). Would not recommend them however, unless you want to run SLI.
Both the i5 and i3 cpu's run on different Intel chipsets but the most popular is the P55.
This should give you an idea of which mobo might suit your build