The Dell 5150 is a Pentium 4 chip and the weight is 8.25 pounds. If you really want a desktop replacement (no use at all outside a fixed location at home or work) then take this one.
Otherwise, if you will at least use the machine occasionally unplugged, especially if you are going to set up a wi-fi (wireless) router network -- which allows you to use it anywhere in the house or dorm or surrounding area - then get a Pentium M based machine (Centrino) of which the Dell 8600 is an example.
Both these machines have 32mb dedicated video graphics which is better than shared video (which deducts from your RAM). If you played more games 64mb or 128mb is better but I have 32 mb in my Compaq X1360US (X1000 series -- 6.5 pounds).
Dell has a 10% off sale right now online as you seem to want to go with Dell.
If you do lean towards a Pentium M chip (Centrino) try to get a Dothan Pentium M (1.7 mhz or above) as it has 2mb L2 system cache now instead of the original 1mb L2 system cache. This cache is what makes Pentium M Centrino machines perform similar to a 2.2ghz or faster Pentium 4 with much better battery life.
Also, if you will use it as a 'laptop' and not on a hard surface all the time, a Centrino machine is much cooler.
As I told you before, look also at the Toshiba M30 series (6.2 pounds) and the Compaq X1000/HPZT3000 series (6.5 pounds) if you want a Centrino machine as they are lighter than the Dell 8600 (Dell has similar components and the 15.4" widescreen but the case was built for a Pentium 4 so that is why it is thicker and heavier.
Also, the keyboard on the Compaq/HP is much better than the Dell (at least the 8600).
It is nice to have a 15.4" widescreen for watching DVD's or seeing more of spreadsheets horizontally. All the Pentium M machines (including the 8600) have this.
But the 15" square screen on the P4 Dell 5150 is also fine and you will see larger web pages most of the time on a square screen (as only certain websites adjust for the widescreen).