Mac V PC is an old debate with many opposing views.

However a lot of the old myths and beliefs are not true anymore.
Years ago Macs used to be more stable, had better sofware and interfaces etc and were superior with a higher bit rate for sound production. Less vulnerable to spyware and other malicious attacks and did not get viruses. Simple answer not any more. Recent times have proved PCs to be as good as if not better than Macs. Mac still have same bit rate PCs have gone higher if you need or want it.

It all boils down to what you are used to working with and personal preferences. Also budget as you correctly say comes into play you get a lot more RAM AND STORAGE for less money with a PC. One other consideration is that most people buy a PC rather than a Mac. They are easier, quicker and cheaper to fix. Less people can fix Macs or will help you with Mac probs. That will tell you something. All the studios I have been in use a PC as a first choice....something to do with Windows programs being more compatable and certain plugins and hardwear not working properly with a Mac.

TIPS IF YOU PLUMP FOR A PC
Re Vista.... ditch it and go for XP. Some people use a PC solely for music production with a special stripped down version of XP specific for music production less unnecessary stuff working in background speeds up PC reduces need for more memory.
I also would not recommend going online at all and using the PC for music production only.
Buy another PC for other tasks, surfing, file storage, burning discs, Emails, etc etc
Also buy a huge external hard drive to store all those large wav files.