I (have to) use Dell laptops at work - wouldn't recommend them to my worst enemy.
They use shared memory with the Intel graphics chip - have a look at
here for some info and a fix.
On my Inspiron 1100 I was able to find a BIOS upgrade that fixed the issue - last week I started to look at the Boss' Latitude D505. No BIOS upgrade, I'll have to set up the 855patch for it.
You probably have the same problem - checkthe Dell site for a BIOS flash, and see if it resolves it, else you're up for the patch.