i7 will work just fine, but they cost an arm and a leg. Their performance is somewhat better than the Q6600, but not all that much - price/performance ratio of the Q6600 is significantly better.

A decent computer shop will build a machine to spec. And it's not all that difficult to do it yourself, anyone who can tell the business end of a screwdriver can build a PC nowadays.

When shopping for a graphics card, be sure to check for heat! ATIs newest  high end consumer cards offer a great performance at a modest price - much better price/performance ratio than the nVidia GX260/GX280 - but they run extremely hot.
They are designed to run hot, so you shouldn't have any problem with the graphics card itself. Unfortunately, the card generates heat very close to the southbridge chip, which tends to get pretty hot all by itself too! Especially nVidia southbridge chips are hotheads.
In a tower case setup, the hot air is getting trapped under the graphics card. So, if you combine an ATI HD4850/5870 with an nVidia based mainboard in a tower case, chances are that you'll blow out your southbridge in a couple of weeks!

Intel southbridge chips generate far less heat than nVidia chips. I strongly advise going for an Intel based mainboard.
At this time, my preference would be a Gigabyte EP43-DS3L mainboard - pretty affordable, very reliable, a no-nonsense mainboard. According to the shop I frequent, Gigabyte boards are currently the most trouble-free mainboards. Asus used to be very good in the Pentium-1 days, a couple of years ago they were definitely NOT good, now they're better, but still not as trouble-free as Gigabyte. MSI also used to be good in the past, but at this time, the percentage of mainboards that malfunction is just too high. Abit is OK, but not as good as Gigabyte.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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