welliam :
- I see this motherboard GA-P55-USB3 and I see it has two PCI one PCI 2.0 and one PCI x16 at x4 what does this mean ?
If you put a graphics card in there it will not run as well as the x16, or x8 slots (which other motherboards have but this one does not). However it could be used for other components as well that require an expansion slot.
- is it possible to install two vga on this motherboard ?
Yes if they are AMD cards you could CrossFire them, but it is not recommended because the performance degradation on the second card, which is running at x4, is so low.
- is it possible to install two vga one of them is sli and the other has sli connection ?
It is not possible to run SLI on this board.
If you want SLI or true Cross Fire, look for a board that has two PCIe channels that run in x8, x8. The card should say that it is certified for SLI. You should be looking at P55 motherboards.
You picked a USB3 and SATA 3 motherboard. If you want a USB 3 and SATA 3 motherboard, these are the ones I could find on newegg right now:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010200280%201070549182%20107172615%201796849719&bop=And&ShowDeactivatedMark=False&ActiveSearchResult=True&Order=PRICE
EXCEPT: Two of those boards do not support SLI/CF. The Gigabyte GA-P55A-UD3 and the GA-P55A-UD3P. There may be others but those are the only two I noticed. Again, if you want SLI / CrossFire, look for one that says something like: 2 x PCIe 2.0 x16 (single at x16 or dual at x8 / x8 mode).
The other thing to note is that if you want SLI / CF and the new implementations of SATA 3 and USB 3, Gigabyte's board makes some sacrifices. You'll see on the link I sent the Gigabyte boards say "Note: When dual graphics cards are used in 1st and 2nd PCIex16 slots, SATA3 / USB 3.0 (Marvell 9128 /NEC USB 3.0 Controllers) will work at normal mode." The following Tom's article states "while those with two cards must live with 2.5 Gb/s bandwidth limits on USB 3.0 and SATA 6.0 Gb/s controllers." The articles follows up stating the sacrifice "is [not] huge or even noticeable on most of today’s hardware". It's a good article to give you an understanding of how it works on the Gigabyte and ASUS boards:
http://www.tomshardware.com/reviews/usb-3.0-performance,2490.html