If you want to get the best out of your battery life you should go for a lightweight setup. Also kernel 3.2 is known to have bettered some of the regressions that happened in earlier kernels.
These are the kernel options I use:
pcie_aspm=force i915.i915_enable_rc6=1 acpi_osi=Linux acpi_backlight=vendor, the
acpi_osi=Linux may or may not be relevant on your dell system.
i915_enable_rc6 enables sleep on idle for the cpu.
pcie_aspm was already explained.
Now to the low power setup. What you could do is take lxde or razor qt desktop environment or you do it as I do. Take some lightweight wm, and the components out of xfce that you need.
I'm up to 7 hours of battery time with my laptop when i turn off wifi.
Now on to laptop-mode. Make sure you have
cpufreq installed, then in
/etc/laptop-mode/conf.d/cpufreq.conf make sure the battery governor is powersave,
cpufreq-info tells you on which governor you are right now.