You didn't say what model you were looking at but I'd assume it's an inspiron, probably the 1520, as I don't think Dell include that 8600 graphics card on other ranges?
If that is the case, you can't do a straight comparison I'm afraid for a few reasons but mainly because the Macbook pro is significantly better built, is far lighter (about half a kilo lighter as a matter of fact), includes some toys the Dell doesn't get (iLife, backlit keyboard, light sensors, magsafe etc) and for one other very good reason:
THE MACBOOK PRO HAS AN LED SCREEN!
Sorry for the (rather unnecessary in retrospect) caps, but that component is currently running at a VERY significant price premium. Heck, Dell charges £82 just for the 13 inch LED-lit screen on their XPS 1330 so you can add in at least £100 for a 15 inch option if it was available. Oh, and that price from Dell only includes a .3 megapixel camera for some reason so that's another tenner on top. Figure a minimum for the LED screen and comparable web cam to the iSight from Dell would be at least £110.
So, let's use that (low) figure of £110, that leaves about £140 to find, right? Well, for a start you can knock off the £55 for iLife if you bought it retail, so we're down to £85. That £85 buys you:
A half-kilo reduction in weight (and corresponding drop in unit size)
Mag safe power adapter
Backlit Keyboard
Light Sensors
Gigabit LAN port (the Dell packs a 100Mb only)
DVI-output with VGA adaptor
Apple Remote
Touchpad with multi-finger scroll (or whatever it's called officially)
Optical audio in and Optical audio out
Choice of glossy or matte screens (the higher res Dells are glossy only)
OSX
Oh, and of course the support of Apple's retail environment if you have the option of using them. Not everyone does though so we can't really include it in the main list of benefits.
Although the Dell does pack in a modem, a couple of extra USB ports, s-video and a memory card reader as standard which the MBP does not. So that needs to be taken into account as well. Still, I honestly think that the above shows that, in terms of a full and accurate comparison the MBP is still on an even playing field with this particular Dell. Whether you agree with that or not is of course down to your own personal opinion.