We're going to need a budget, what games, what framerate you'd want, and what resolution you're playing at. And your current computer specs would also help.
To give you a proper budget we'd need your system specifications (go to [link] and download the program, then run it and paste the "summary" here) your budget, your country, and your intended use (what videogames do you play, do you do a lot of video editing or something like that, etc.)
soemthing like an I5 3550 or something with a GTX 660 Ti or AMD HD 7870 8 gigs of ram at 1600Mhz a 500 watt + PSU from corsair, 1TB hard drive from WD or samsung and a corsair 200R. And an SSD as a boot drive if you can afford it. That should make for a good PC.
Before asking for recommendations for good computer equipment, you should first state what your intended budget is.
Otherwise, I can easily just suggest the super high-endest of the end hardware which might as well cost the same as a house.
Otherwise, just Google the stuff. There's plenty of sites which provide insightful information (read:graphs) which would far exceed what can be illustrated in these forum posts.
The graphics card is the highest priority if you want to upgrade anything. RAM and CPU are also important, too, although you'll probably have a harder time upgrading the CPU than the RAM