nothing like building ur own system, but as the price diffence is minimal id say go for the alienware, looks cool, more likely to have better component compatabuility. and tech support.
oh and u can get some matching kit like keyboard...
it really depends how u optomise the quadro, if u do it correctly, then u can make it run games quite well. But the quadro should be able to pump out more pollies.
check out the alienware section jclause is very good with drivers.