powerule, if all you're looking to do is webdesign/graphics work... then the old x300 128meg out of the I6000D will be all you need in the I6400. And if you'll be doing more web than graphics, you can suffice with the Intel GMA950 even to save a few more bucks. The 100$ you'll save over the 256meg graphics will pay for an additional battery for you, which sounds like it'll be better for you since you're concerned more with mobility and battery life.
You should get similar battery life out of the GMA950 and x300 options. The x1400 may use a lil more battery than the other two, but not much. The I6000D actually had the longest battery life using dedicated graphics over integrated graphics.
As for helping maximize your battery life, go into BIOS and set the screen brightness as low as you're comfortable with. This will have a noticable impact on battery life. As will the types of applications you run. If you get enough physical ram (2gb) then you can disable virtual memory. This will make all your applications work on ram instead. The decreased hard drive usage will extend battery life noticable while "working" on the machine.
Also, avoid mobile DVD watching unless you only want to watch 1 movie. You'll get about 2 hrs of movie watching from a DVD because of the amount of power the optical drive draws.
And last, streamline your XP installation. This includes reducing background processes in the Services Administrator's Tool. Doing this will lower the about of weight on the CPU and will lower power requirements also.
That's about all you need to know to get the most life on battery.
And to keep your batteries as long lasting as possible. Remember to cycle them properly. This involves charging the battery all the way up, then booting up into BIOS, leaving the machine in BIOS setup, unplugging the power cord, and letting the machine run in BIOS until it runs out of juice. Do this twice in a row every other month to make sure all the cells are properly working and charging to full capacity. No matter what they say, no battery is "memory free".