I like gigabyte boards, But I would pass on this one - It depends on current and FUTURE use of the PCI-e slots. This one is very limited.
Quote from specs
1 x PCI Express x16 slot, running at x16 (PCIEX16)
*For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.
1 x PCI Express x16 slot, running at x4 (PCIEX4)
*When the PCIEX1_2 or PCIEX1_3 slot is populated with an expansion card, the PCIEX4 slot will operate at up to x1 mode.
Some of the other MBs do NOT spell out limitatings, so I would down load a manual on a board that you are thinking of Buying and comb thru the manual so that limitations do not bite you later.
I think your memory would be fine. One note. You bought 3 sticks to run in triple channel mode (correct). The P67 boards are dual channel so you would probable want 2 Pairs - could probable buy a single stick (SAME specs) for 8 gigs.
Yes You can use all three sticks. But for Dual channel memory needs to be installed in pairs. One pair, or two pairs = daul channel. 1 stick, or three sticks would operate in single channel mode. What you could do is install just two sticks (4 gigs), then catch the memory on sale and buy the "4th stick" and then install the 2nd pair.
With the possible exception of photoshop you will not see much diff between 4 gigs and 6 gigs of ram. Bigger performance hit would be in going from daul channel to single channel mode.