For the archives...
My performance improved drastically after following the advice in this thread
Wireless connectivity, XPS M1330 with Intel 4965 Mini Card - Notebook Forums and Laptop Discussion
In my case I'm using XP, but it seems like 802.11b performance (or not) depends on the right Dell/Intel driver. This driver may be an
earlier version (i.e. downgrade, see thread above for details)