Another good company is PC Specialist. Don't be put off by the rubbish name or by their rubbish adverts in the specialist press, I gave them a whirl and they were superb. Great prices, even better delivery times, and well packaged upon delivery too.
As far as graphics cards go, just look at THG charts and you'll get an idea of how good stuff is relative to each other, then just pick one in your price range. I love my i7 with 12GB of RAM, but it doesn't get used for gaming at all and you'd be better off swapping CPU power for GPU power if it's a games machine. Don't get sucked in by pretty cases either, spend that extra money where it counts like RAM or a better PSU.