I always buy my pc's / laptops online.., even from Ebay but always always check the feedback/reviews. In this day and age u can have as many problems from somewhere like PC world fulfilling their warranty (heard some horror stories, even been on Don't Get Done Get Dom in a very unfavourable light).
In my experience, nothing wrong with buying from a reputable online seller as long as they ARE reputable. And I find for the same price u'd pay for an out of date spec laptop/pc.., u get a lot more for your money and so the laptop/pc works a lot longer. If I had a pound for every time I have had to tell a person to get more RAM cause in spite of them being told in Comet/PC World/Dixon's that the pc had enuf Memory to run Office properly.., the pc/laptop only had half the amount of memory it needed to run anything without having a fit every five minutes.., I'd be a very rich person (and same goes for graphic cards).
U can get very good deals, but to be honest before buying from ANYONE whether online or a shop.., u should have a good idea what u need and why. Otherwise, anyone whether an Online retailer or shop will just sell u what they want to sell u, which is often not what u actually need. Sales people are there to make money for their employer and themselves, normally, not to do u a favour lol. And I know this may shock u.., <evil grin> they don't always tell the truth!
Apologies if I have caused offense to anyone.