Well, it all depends on whay you want for your money.
For example the XPS 1530, with the same processor as the macbook pro 15" standard, then it comes out at about £999 (a lot cheaper than the macs £1299, AND you get more RAM (for £30 more you can up it to 4gig, mac costs an extra £120 to get it to 4 gig), much larger HDD, and a blue ray drive, BUT its got a lower screen resolution
If your after a larger on 17" then the Macbook costs £1799, the XPS M1730 with the same processor and a larger HDD (the choice is 200GB or 320 so went with 320 as its the closest to the Macs 250 gig) BUT the graphics had 256meg compared to 512 on the mac but with a larger screen res than the mac comes out at £1570 (you could always lower the HDD and the porcessor speed and add a blue ray drive for a similar price).
If you wanted to get as close to the MAC's price with the Dell XPS M1730, you can get one with the same 2.5gig processor, 4 gig Ram, 320 gig HDD, DUAL GeForce® 8800GTX 512 meg cards for £1750 compared to the single card, 2gig, 250gig macbook.
So on the basis of the Dell range its cheaper, and withthe case of the 17" models you get more if you push the Dell up to the Macbook proce range.
Have a look at other manufacturers sites to fit your needs.
"No pressure, no diamonds" Thomas Carlyle