Here in Canada, the best deal by far on a "new" desktop computer is a refurbished HP Media Center machine on sale. You cannot even build one yourself for less money. These HP machines also use solid good name brand components such as ASUS motherboards, and Seagate hard drives. By refurbished, they simply mean that the computer has had to go back to factory for some reason. If the box is simply opened in the store, then back it goes to factory for refurbishment. By factory, I mean an authorized outlet for examining/troubleshooting the machine. These machines are also current, meaning that they are not "lease returns" that have been used for a couple of years.
So where do you find such machines? Futureshop and Best Buy both carry them on a regular basis. But like I said earlier, wait for them to go on sale, where you can scoop one for less than half the price of new, saving you upwards of $500. Yes, it is well worth paying the shipping charges.
Also, the refurbished HP Pavilion and the Compaq machines are also good deals, but nothing beats the sheer value of those Media Center machines with their better graphics cards and hdtv tuners built in, and typically huge hard drives.
The only downside I see with buying refurbished is the shorter warranty period of 90 days. But misbehaving components usually show up within the first 30 days anyway. Also, HP should be using beefier power supplies in their machines.