Depends how hardcore you want to game. If you want a Ferrari, you'll have to shell out big bucks, but you can game on a budget as well.
I recommend the ASUS F1 series as a starting point for your motherboard.
Always good to go with Quad Core nowdays, with something over 4g of ram of memory DDR3-1600
I use a Radeon HD 7770 videocard and I can run all modern games at at least 30fps and higher. You can check here for price comparisons on cards, and what's right for your budget.
GeForce is probably the way to go as it's newest models are top of the line for highest quality.
make sure you have a good accelerated processor unit.
That's about the bread and butter. You can look up the parts on newegg, buy them seperately and assemble a rig yourself which will save you money, or scout for some deals in your area. Unless you buy something second had, you're looking at $400 as an entry point for anything remotely decent, if you want the cream of the crop, you can head into the $1000+ pricepoint. PC gaming is awesome, but it certainly isn't cheap. As a long-term investment, it's a better bang for your buck than consoles, and you get insane deals on Steam