I'm building my first gaming pc and all I really need to figure out is the graphics card. It would be nice if it was under about $90 but if t means getting more performance then I will pay $100.
You're not really going to find something of higher performance for that price. It should be able to handle most games at low-medium settings at 1080p.
the hd 7770 is probably the best sub $100 gpu you can get.
However if you bargin hunt a bit you might find a 650ti or 7790 for under $100, and they'll be worth every penny more you spend... you'll see a pretty nice boost in performance (they're both about 20% faster then the 7770)