One of the first questions you come across when examining computers for music making is whether to get a laptop or a desktop. The desktop is far easier to upgrade if you plan to keep it a while, while the laptop allows you to move around and you can get one that is almost as powerful as its desktop counterpart.
If you plan on staying in a studio room in your home choose a desktop because you can keep upgrading it to keep it current, this will get you more use for a cheaper price. It also has more connections for external hardware than a laptop, making it easier to integrate with your mixer, external hard drive, preamps, and other things you may need.
The laptop will allow you to record live performances more easily, take your recordings with you, and may be better if you have to set up and tear down your studio space frequently. A desktop offers more connectivity with less flexibility and a laptop offers a little less connectivity with a little more flexibility.
Despite the marketing claims, there is very little difference between a Mac and a PC for music making. The Mac has been the industry standard for a long time but today you can do the same on a PC as you can with a Mac. Macs run very smoothly and are not prone to the viruses that can slow a PC. They also come with less software pre-installed than a PC and the operating system is better optimized for music making, although you can customize Windows to work well.
Macs are also significantly more expensive than their PC counterparts, so the choice should really be personal preference, rather than ability to make music. The Mac also comes with Garage Band pre-installed, which could save you having to purchase another music recording program.
Processors today have all sorts of options with multiple cores and different speeds. Basically, for music making, faster is better. Most music making and recording programs support multiple core processors and are able to take advantage of the extra speed, although you should check your music programs for compatibility before you make your purchase.
Music recording and creating requires a lot of disk space and most computers will have plenty of disk space. However, the speed with which the computer can write to the disk is very important for riding high quality audio. A 7200rpm disk is a good investment and most computers only offer a 5400rpm disk as standard, so its worth shopping around for one. The faster the disk can be written, the higher the quality of audio you can record at one time without it popping, dropping out, or skipping.
As with processors, the more memory the better. You should have at least 4GB but 8GB is better if you are able to get it. This means that the computer can process the audio more quickly and do so without popping, dropping out or otherwise messing up your carefully recorded tracks. They also allow you to run more applications at once, which makes it easier for the computer to handle any tasks that may be happening in the background as you're recording.