The laptops are both £539, which is the absolute limit of my spending. The Inspiron E1705 seems to be at least £1300.
Right now I have a Celeron 1.6, Intel GMA 915 and 512MB RAM and Vegas is OK (even if render time is long, which doesn't bother me), 3DSMax is OK with LFS cars (again, long render times) but I'll need something more soon. I don't need a 'gaming' laptop, just one that is noticeably better than what I have now.
As for the screen size, 15.4" wide is enough as it has to be portable.
I'm leaning towards the Inspiron 1501 as it has more RAM and the graphics card is in the same class as the Inspiron 1521.