MD are cool about you upgrading yourself, I've bought 5 pc's over the past 5 years or so from them and not had any mither with returning things (a rarity as most of their stuff has been spot on)
I'd just avoid Maxtor HDD's over 40gig, as I had bad sectors on 3 Maxtor 80 gig drives I got, 2 of which were intermitant which is a pain in the arse when you take them in for testing and they say "they're fine!" then you get them home, and they fail (proper style) 2 days later.
Seagate drives are a much better build and more reliable.
Other than that, go for it, best place for pc bits in my opinion
