I was in the same situation as you, except I wanted a gaming machine also. I looked at both the Mac's and Alienware (which I ended up buying). Mac's are great machines, don't get me wrong. I would get one at the drop of a hat if it wasn't for all the hassel to get a Mac machine to work with other windows based machines on a network. That and the Mac operating system takes a little longer to get used to, especially if you have been using windows all your life. The only great thing about Mac's is that if you do get one, you will never have to worry about a virus. The newest operating systems for Mac's are based on Unix, which so far no one has been able to code a virus for (or at least a succesful one). Mac's also have some pretty impressive graphics, that and over all their battery life is a lot longer than a comparable windows based machine. The reason for that is because they require less resources to run. For example, the newest Mac's processors run at around 1ghz (I think) and that is more than it would ever need. Heck, it could probably run on a 800mhz system and still be as fast as a windows system running on a P4 3ghz.
As for Alienware systems, they are well, among the top windows based systems. My system has a P4 3.2ghz, 2 gig ram, bla bla bla... it can run Maya and Photoshop (all in the process of doing something, not just sitting there) all at the same time without even any noticable slow down, no joke. I got the creative professional version of the Area51-m, the Roswell-m, becuase I could get the NVidia Quadro Go 1000 graphics card. That is the reason it can do that. That graphics card is probably one of the better (now out dated) AGP cards for laptops. I run UT2k4 at maxed settings in online play with no lag at all. It is one serious machine. I don't think you could do that on a Mac. If you can, please show me the set up that can. Heck, my chem prof is a Mac-aholic (sorry, bad pun) and he looked at my machine as was very impressed. It actually has better graphics capabilities than his Mac laptop does. An example of this is a molecular modeling program we use to view molecules (like a strand of DNA) in 3D, the view on my machine is smoother and a clearer picture than his. The one on his looks jagged and runs slower. However, the down side to Alienware is the battery life. It leaves a lot to be desired. I just carry my power cord with me everywhere, but even that weights a ton. Even the power cord has its own cooling fan!
So, I guess that should sum things up pretty good, Mac's are great for long battery life and decent to awsome graphics (the desk tops have much better graphics than the laptops do). The Alienware have aswome graphics all-round and extremely powerfull but are severely lacking when it comes to battery life. I don't think I can ever get 2hrs out of my battery even just listening to music, it usualy dies after an hour and a half. I have the 5500 which is an inch smaller (less thick) than the 7700 and it is pretty portable. I can carry it in my backpack without too much of a problem. I don't even notice it anymore when I am carrying it. Mac's are much smaller and a lot easier to carry (smaller as in thinner).