I'm a bit out of the loop with DSLRs at the moment as I've gone back to film and my Canon is coming up to 4 years old.
But, I would say when you are choosing the camera, go and try it out first. By all means buy it online and get it cheaper but have a play with some in Jessops/Currys etc to get a feel for the differences between the manufacturers.
Secondly, dont get excited about megapixels - you are better off and will get better pictures by buying a cheaper cameras and more expensive glass (lenses). My Canon is 10MP and I've never had a problem blowing the images up and getting them pro-printed even at 16x24" - and it is rare that I do that!
Thirdly, look the "system" you are buying in to. Factor in the cost of buying additional lenses, flash guns, tripods, filters etc as and when. It might be better to go with the more expensive camera maker is the extras will be cheaper in the long run.