I suggest a HP Probook with LED and no glossy screen, simply because its one of the few that has non glossy screens. If your used to OS X I suggest you go with a Macbook air or pro, but if you only need it for IM why spend more money than you need to? I`m not trying to start a apple vs. microsoft discussion but in MY opinion the big difference is design and price.
I use an HP desktop, i3, running on Win7. I would suggest you get a computer running on the latest procesor with plenty of RAM and harddisk space. It's hard to keep up with technology these days so if you have the money to spare, better get the latest in greatest because in the next few months or years, that would be considered old.