Building new PC next week. Last minute questions.

My laptop crapped out on me again and this time it's not worth fixing. Going for a budget build immediately, to upgrade over the rest of the year. Ordering parts tonight or tomorrow. Couple quick questions, as it's been a while since I've built one.

1: if a video card says its interface is PCI Express 3.0, does the slot on the mobo have to be 3.0, or will 2.0 work? If it works, what's the disadvantage? Just doesn't run at full speed, I'd assume?

2: Any suggestions for a good but reasonably cheap wifi card?

My motherboard is an ASUS M5A78L-M/USB3 which has PCI-E 2.0. My 4gb MSI GTX 770 is PCI-E 3.0. No problems here.

PCI 2 is fine, there's almost none performance boost comparing those two:

About the Wifi card: http://pcpartpicker.com/part/tp-link-wireless-network-card-tlwdn4800 this one seems popular (if you can't get this module try keeping with good brands such as: TP Link, Edimax, Linksys, D Link)