The Gigabyte board is a good board but doesn't support SLI or even really Crossfire with the x16x4 setup.
If you are only planning on running a single card setup (never plan to run dual cards), I would go with the Gigabyte board as it is the cheaper board. If you think you might one day run day, I would go with the ASUS P7P55D-E Pro.
Getting the i5 750 is a great choice for a CPU, so you aren't going wrong there.
Sticking with the GTX 275 isn't bad idea either. It is still a very good performing card in gaming. I'm running two GTX 260 Core 216's and don't plan on updating until the next gen GTX 4xx series cards come out. I'm not impressed with the GTX 470 & 480 when it comes performance to price.
I say stick with the i5 750, GTX 275 and go with the ASUS P7P55D-E Pro but of course it isn't my money to spend
Ya that`s ok, it`s just that I couldn`t find the P7P55D-E Pro in the site I want to buy my items.
Let me explain my situation to you hehe, I`m brazilian and will travel to USA during my vacation. Then I don`t want to shop my items from Newegg because it`s online only. I intend to buy everything with Fry`s so I can pick up my item at the store and check them up before taking them to Brazil.
In your case, I would go with the Gigabyte board, if available at Fry's, since you don't plan on running dual cards. It will provide an upgrade path with USB 3.0 & SATA 6.0Gb/s support. If the price is better, you can get the ASUS P7P55D Pro and still be okay.
The only thing to be wary of with the GA-P55A-UD3 when using a single graphics card in the PCIe 2.0 x16 slot is that when using any of the USB 3.0 or SATA 6Gb/s features, the PCIe slot speed drops from x16 to x8.
Here's the Gigabyte specification page that lists the note as:
"When SATA3 / USB 3.0 (Marvell 9128 /NEC USB 3.0 Controllers) work at turbo mode, 1st PCIE x16 will work at x8 speed."