Integrated graphics are designed with the "just enough" philosophy in mind in terms of performance, but deliver on the power savings. Discrete graphics are usually much more powerful at the cost of battery life.
Were you considering a particular system that you need help with?
Integrated graphics are always part of the chipset and as such are extremely limited in scope, dedicated always have at least a separate chip so you get more space to cram in transistors to do graphical work. Keep in mind one common misnomer is that discrete graphics can be upgraded, that's rarely the case in all but the highest end notebooks that use the MXM standard or a similar propretary standard for their graphics. Also discrete graphics tend to improve resale value as well as sometimes just the ability to resell at all, so if you're expecting to upgrade in the not so distant future buying the cheapest integrated solution may not be the best in the long run when you want to "trade up"
rflcptr, I was thinking of getting rid of my desktop system (c2d 6600,8800gts,2gb ddr2) for a laptop instead since I usually travel every other weekend. There are two systems that I was thinking of getting which are: