1) Well, there are a few other high end computer companies out there, like Falcon Northwest and VoodooPC, but 3000 dollars is barely enough to scrape the bottom of their laptop lines. You can check out Gateway if you'd like, they're making the move into the gaming PC sector with their FX series. I'd almost suggest against considering Alienware, I've heard mixed reviews about their customer support.
2) If you can save money by getting your own RAM, go for it, since it's so cheap now.
3) Yeah, there's a noticeable difference between a 7.2k and 5.4k RPM hard drive, at least in my opinion. If you're willing to wait longer for loading, then don't bother, but in most laptops, it's an annoying bottleneck, because you can't overclock a hard drive, and it just slows load times. Luckily they're easily upgradeable.
4) If you're extremely careful with your laptop, you can probably swing by with the 1 year, but then again, if you're covered by warranty and your hard drive decides to melt, the company will replace it for you. If you can afford it, get the higher warranty, I think at least. It's a precaution, and some peace of mind.