my answer would be ... there isn't a good lappy for gaming.
.... not if you are hardcore anyhow
LeMarchand and dredwerker are both spot on ... the real issue is the graphics card ... and the fact that you can't upgrade a laptop
(not true - but only a couple of verrrrry expensive models)
hard discs . I think the largest HDD for a lappy is still about 160Gb - you can go external .... but no good for gaming (bioshock just ate 5.5Gb of my disc)
for a desktop you have unlimited internal storage space .... well ok slight exaggeration .... generally 4 or 6 bays which will each take a HDD (max 1Tb each - if you can afford)
pound for pound a lappy is far more expensive that an equivalent desktop