No, it won't void your warranty. It's a user interchangable part, and Dell even has a dedicated service that sells upgrades/replacements like that. As long as you buy the parts from Dell, you should be fine. If you'd like a little more assurance, just call them and tell them that you're looking to buy and you'd like to know if you can upgrade your drive later through them, and you want to know if your warranty will still be good. Shoot, ask them any question you have while you're on the line with them =) I deal with the Education/Government side of them, which would be a different customer service center, but I'm sure they're as helpful as those that I deal with... or at least should be =) I suppose there's always a couple jokers in every deck though =)
If you're going to buy the best laptop you can afford at the time, I'd up for the best warranty you can afford too =) I'm pretty clumsy and hard on things, so I opted for the 3 yr. Complete Care service, which did cost me a few hundred extra, but it's almost already paid for itself and I'm only at 10 months into the 3 year contract! =) Those pesky laptop parts can get pretty expensive =) Dropping my battery on the cement floor, and so on is kinda hard on it =) I did that a few months ago. When it did happen, I just called them up and said "Hey, I broke my battery on accident when I was pulling it out" and they said, okay, asked a few simple questions and shipped a new one right out to me. Kaz likes Complete Care coverage... Kaz needs it =)
You might not though, that'd be up to you and your unique needs.
Later,
Kaziah