Video card is the most important part of a gaming PC. The more you spend, the higher quality graphics. I just purchased an EVGA GTX780 2 months ago and it has been great so far. I'm still running an ASUS P6T with an i7 940 processor with 6Gb or RAM (built in 2009) and it is still more than enough for gaming.
Priority:
video card
processor/motherboard
ram
hard drive (helps faster load times)