I would suggest going with the E4600. The only real difference between the two is that the E6400 has a higher front side bus, but for moderate gaming and family use the E4600 is plenty.
Also, as far as a power supply goes, just look for something between 450-500W. That should give you all the power you need and then some.
And for the motherboard, look for a P35. I have found one on Newegg for you that is inexpensive, well rated, and will do everything you need it for.
Newegg.com - GIGABYTE GA-P35-DS3L LGA 775 Intel P35 ATX All Solid Capacitor Intel Motherboard - Retail