you can upgrade the harddrive and ram. other things i'm sure, but these are the most common ones.

you sure about that?

I've read a lot about PC's being buggy with certain sequencer programs and MIDI, but I've been using a PC with SONAR and soft synths flawlessly for years. There's always a learning curve with any program, so it helps to at least understand the operating system your running before trying to learn the software (that is, if you're not already familiar with Mac OSx, there will be a higher learning curve to finally get music recorded).
I think you may have more software options with a PC, although that's changing now that some Macs with Intel chips can run Windows (Of course, why not just get a PC if you're planning on using windows software?)