I always find that if you give them a call you get better discounts. Call them in the evening and I have found they will knock loads off the price as they want to meet their commission targets.
Just got a really good deal for my mum - pretty high spec with 19" widescreen monitor. I told them the most she could possibly pay is £500.00 and they knocked about £100.00 price off the internet price. If they won't knock money off talk them into including extras/higher spec.
Definintely worth haggling. If you don't get anywhere with the 1st person you speak to, hang up and try the next one! Trust me it works
Cheers,
Natalie