i found the Asus P5P43TD/USB3 motherboard to be perfect for what I want. But now I'm thinking that I might want to use 2 graphics cards if I want to use dual monitors( or can i use 1 graphics card for 2 monitors?), is there a motherboard that has usb 3.0, supports up to 16 gb ddr3, and has 2 pcie x16 slots?
You can use a single graphics card for dual monitor setups. All modern cards have at least 2 connectors on them. Depending on your card, there will be a combination of 9-pin Analog (DSUB), DVI, or HDMI. You just need to enable the use of 2 monitors from within the control panel (Nvidia has their own control panel, not sure about ATI because I don't use their cards).
I am fairly certain there are no 775 boards that support DDR3 use. Any board can be made to use USB3 using a PCI-e addon card though, and the majority of motherboards will offer multiple PCI-ex16 slots.
Well I found a computer for a GREAT price that has an intel core 2 quad processor which I will be using in my new pc. But if i can use a single card for 2 monitors then I guess I wont worry about finding another mobo.