Regardless of the brand, Kingston or G-SKill, the P55 platform uses either 4 or 8GB (or 16), at least if you want to run your memory in dual channel (which you want).
Now, if you never plan on running 2 cards in cf/sli you would do a good choice with either the Gigabyte GA-P55-UD3R or the Asus P7P55D. These both come in versions with Sata III and usb3 as well (A and E suffix), but i wouldnt recommend them since the Sata III implementation is subpar and usb3 can always be added by means of a pci-e card if necessary. Both mobo's have a decent bios with enough overclocking options, good memory support, easy flashing and possibility to store profiles.
If you do want to keep the cf/sli option open I would recommend the Gigabyte GA-P55-UD4 or the Asus P7P55D-Pro. The Gigabyte is almost perfect, except for the fact it has only 1 software controllable fanheader (same goes for UD3R). The Asus is good as well, except it has a little higher power usage and lesser quality soundchip, but you'll probably need good ears and speakers to hear the difference. There are Sata III/usb3 versions as well for these (the Gigabyte is at a slight disadvantage here since it sacrifices pci-e lanes to run Sata III/usb 3 at their maximum speeds).
MSI P55-GD65 is a fairly good alternative, except it's a bit lacking in the memory compatibility department and doesn't have the option of using dynamic vcore when overclocking (unless it has been added by means of a bios update).
More expensive motherboards generally don't really offer more performance. They usually do have extra connectivity (more sata ports, firewire, dual lan ports etc) or beefier heatsinks.