I'd say don't build the PC based around the keyboard and mouse. Just add that stuff later and get all the PC you can for now and run with a crappy generic mouse and keyboard or you're talking $100 out of your budget meaning the PC you're looking at is really in the $600-$650 range. That can be quite a difference when dealing with these price ranges. I just saw a decent combo deal on new egg with solid components for around $760 shipped:http://www.newegg.com/Product/ComboBundleDetails.aspx?ItemList=Combo.573823
I was pretty impressed when I saw this combo deal.
Pick up the mouse and keyboard over the following months. You'll be so much happier than if you would have compromised on the machine just to get that particular mouse and keyboard. The mic is another thing you can just pick up.