First, I am not an expert on this topic. CMU buys me a new machine about every 4 years, and I do not keep current with the industry. I've been a PC user for many years (although many of my collegues use Macs). CMU is neutral on this subject. Almost all software that we use runs on both machines (Macs run a special version of Unix as part of MacOS; many SCS machines run Unix as well). So, on the PC vs. Mac question, I make no recommendation. FYI, about 90% of the incoming SCS freshman class has PCs; about 10% have Macs.
Next, no student needs to buy a computer to be a CS major at CMU. We have many labs, located everywhere from academic buildings to residence halls (some maintained by SCS, some by campus-wide computing services). Common file space -allocated to every student- is easily accessible from all these machines. Some of these machines are very powerful/special-purpose, e.g., in graphics labs -machines that you will have difficulty duplicating.
This said, our standard machines are typically 1-5 years old (most labs are on a 3-5 year replacement cycle) and many students find it convenient and productive to purchase their own computers and use them in the privacy of their dorm rooms (or if portable, in classes or other places on campus). You will be able to connect to the campus network easily, either through an ethernet or wireless network (which serves most locations on campus).
If you decide to buy a machine, remember that it will probably be a 4 year -not a lifetime- investment. Strive to buy something with reasonable power at a reasonable cost; don't drive yourself crazy trying to get something optimal. FYI, I recently bought a laptop for about $700 (1.5GHz processor, 512 Mb memory, 30 Gb hard drive). I also recently bought a desktop for $2500 (3.8GHz processor, 4096 Mb memory, and two 250 Gb hard drives).
Before you buy anything, check out the CMU Computer Store I am not advocating that you buy your machine from them, but you can see their standard configurations and prices, and then compare them against other sources.
I think the number one mistake that students make is buying a processor that is "too fast" and buying too little memory. You pay a premium that is outsized for getting the fastest CPU: often one that is 5%-10% slower costs much less. It is better to get a bit slower processor and more memory (too little memory can slow down an application even more than a slower processor). Computers are very fast: often, their speed is severely limited by how fast the user types!
Data from the Freshman Survey shows a median system comprising a 2.0 GHz processor, 1024 Mb memory, and a 80 Gb hard drive). I believe a sysem with these specifications would support students nicely for their undergraduate years here. Think about these machines not being used so much for heavy computing, as for writing papers, connecting to the internet, etc.
To me, the most important parts of the computer are the ones that act as interfaces: the keyboard, screen, and mouse. So, the most important decision you will probably make concerns buying a desktop vs. a laptop. I've found it difficult to type on a laptop, and I need a big screen. I cannot use the touchpads effectively, so I just plug in a mouse whether on desktop or laptop (caution, some of these issues might relate to my advanced age, and not be relevant for freshmen). I rarely use a computer outside of my school/home offices, so I have desktops at both (and a cheap laptop for occassional uses). I have collegues who have expensive laptops and no desktops; some have docking stations on their desks for their laptops. De gustibus non disputum (there is no account for taste).
Well, as Forrest Gump said, "And that's all I have to say about that."
Rich Pattis, Summer 2005.
PS: Apple will be switching to using Intel's x86 chips "soon". The result is that eventually the current Apples (with PPC chips) will not be well supported (in terms of new software and software updates). Some folks think that now is NOT a good time to buy an Apple; others think that for a 4 year investment, buying now is OK. A big indicator will be whether Apples sales plunge in the next 4 months (in which case it will accelerate the migration to x86 chips; if they don't plunge, current Apples will probably be supported longer). Apple generally has a reputation for not supporting its older products (which some argue allows it to move ahead, unburdened by older technology).