Well, recently I've built my first gaming rig, and as excited as I was, I zoomed past some of it. I ended up getting a Micro ATX board, instead of a normal one. Currently I have my graphics card in the 3.0 slot and a wireless adapter in the PCI-e. Is there anything else done the line that I'll run into that necessarily needs one of these? I don't really plan on doing an SLI rig, I'll just buy a better graphics card.
Almost anything you can get in PCI-E, you can get in a network version or a USB version, so you probably will be fine. IE, most likely things to need a PCI-E slot for would be Sound Card, RAID Card, or TV Tuner card, all of which can be had in USB versions, or over the network (IE, get an NAS instead of a RAID Card, etc).