Heavy depends on useage and on the person. I gladly lug around 7+ pounds of ThinkPad because it's a small price for me to pay for the ThinkPad's legendary robustness, plus I use it in leu of a desktop. It's only 800MHz, but two and a half years ago when I got it, the thing was a speed demon. 800MHz and 256 MB RAM, and it handles everything I throw at it, so I think that just about any laptop today will be good. I've run 5-6 IE windows, Outlook, Word, Trillian and Winamp with Photoshop in the background and there wasn't much slowdown. If my old machine can handle that, I think you'll do fine with almost any new machine.
ZV