OK here's my 2 cents (I work in tech support and network administration and have my A+ certification if that means anything).
For novices, I recommend Dell, their service is second to none. The systems are a bit more expensive than the other brand names, but any problems are dealt with by an at home technician (they go to your house) within one business day for the first year of ownership, and extended warranties are available for up to 3 years. Hardware is generally brand name, intel processors, western digital hard drives, brand name cd/dvd drives, brand name memory, etc. Compatibility is no problem as well, parts are interchangeable with other clone computers, which is a major problem with the other big names (HP and Compaq for example).
If you have a reasonable knowledge of computers, go to Computers and Peripherals at Steeles and Keele, or Canada Computers at Spadina and College. They can customize a system based on what you are looking for (eg. gamers, heavy internet users, graphics users, etc), with brand name parts, and their technicians are all A+ certified and know their stuff, if you have a problem there is no red tape like at a Future Shop (where no one there knows anything about computers so they have to send it off somewhere and you won't get it back for a week or two), you just bring your system in and the guy will look at it right away.
My first computer was from Ultinet at Dufferin and 401. What a piece of shit it was, never shut down properly, all parts were no name, crashed constantly, service was crap, I had to bring it back 4 times in the first 6 months. I've heard MDG was pretty much the same, crappy parts and service, they depend on cheap prices (which come from shitty parts).
In short, don't base your purchase strictly on price, you do get what you pay for.