At risk of a flame war ....
I absolutely have to recommend against Dell (despite using a Dell screen, Dell E6400 laptop and Dell keyboard to write this....)
All our work kit is Dell (we moved from Acer and Toshiba for political reasons...) and the fail rate stats went through the roof! I mean literally - we're talking 150% higher than before. We pay for NBD 3 year cover and we had to start getting the users to phone Dell directly to raise call outs because we were spending all our lives on the phone to them! Thats been across 3 models so far - D630's, E6400's and the new E6410's (which I must admit are very quick!).
Personally I'd always buy HP - I've never had one fail yet.