It sounds like you will have the desktop perhaps for DVD burning and other heavy tasks and you want a lighter notebook to use away from the desktop. Also, if you have DSL or Cable internet at home and already don't have a wi-fi router it is great to leave the desktop hard wired to your connection and then have the notebook and/or other computers in the house using wi-fi.
*** The first priority is for you to decide the LCD size you would be happy with and also which cpu family you will go with.
A Pentium M (Centrino) notebook sounds great for you as it will have maximum battery life, weighs much less for whatever LCD size you get (versus a Pentium M and to a lesser degree an Athlon 64) and it also runs cooler if you do occasionally use it off a hard surface.
Most consumer notebooks are 15" or 15.4" widescreens. The Compaq X1000 series I have (same as HPZT3000) weighs 6.5 pounds with a 15.4" LCD. Another great option for this size is a Toshiba M35 series.These will fit in a student notebook backpack easily.
If you want to go smaller, you can get 14.1" notebooks through many business lines (HP, IBM, Toshiba) but they are actually more expensive for what you get as they are business. They weigh about 5.5 pounds.
You can go smaller than that -- 13.3" Fujistu, 12.1" Sony, etc -- but then you often are getting Integrated video (shared video).
If you really will only be doing web surfing/Microsoft office, etc that might be ok but for an all around notebook that can do more you want dedicated video RAM (shared uses system RAM -- dedicated has its own video card video Memory).
You cannot upgrade your video memory later so this is an important choice to make.
As far as pricing, you can easily get a nice Pentium M (Centrino) notebook 15.4" widescreen like the HPZT3000/Compaq X1000 for between $1,000 and $1500 after discounts and rebates.
See hpshopping.com and X1000forums.com for details (here you can see exactly what people are ordering and what discounts to get).
For example the HPZT3000/Compaq X1000 currently has a $100 instant rebate, at least a $50 mail in rebate (possibly $100), a free upgrade from DVDROM to CD-RW/DVD ROM, etc. This is at hpshopping (custom order).
Toshiba has 10% off all custom order notebooks at its website.
Keep in mind that there are great prices both online and retail right now as it is the end of the back to school (2nd largest notebook selling period) so get one soon if you really want one before Christmas time (#1 selling season).
Apple notebooks are also a great choice for some people but you probably have an IBM PC desktop already.
Also, most notebooks have a 1 year limited warranty but you can double that by using an American Express or qualified Visa/Mastercard Gold Card/Platinum card that has electronic warranty doubling (up to 1 year).
Also, by ordering custom you can get a faster rpm hard drive (5400 is 50% faster than 4200), you can buy less memory and put in more yourself later to save money (only 2 memory slots on notebooks so only order RAM in 1 slot), etc.
But decide first what processor type and what LCD size you want first and then the rest falls into place.
I currently prefer the desktop PC, but am looking to purchase a lower end notebook to do basics (e-mail, web browsing, a little document and spreadsheet stuff, etc.) while occasionally away from home PC.
With that said, I obviously don't want to spend $2000 on a high-end notebook. What should I be looking to purchase? Or better yet, what specs should I stay above to stay useful (not cutting-edge) for the next year or two...