It really depends on how powerful or recent your Mac is.
Broadly, you can run Windows software on an Intel Mac, otherwise a program such as Virtual PC will allow you to run Windows software on a PPC Mac.
A good spec machine should be able to run VPC without any loss of performance
MYOB is a good comprehensive medium-weight package for the Mac (I use it). The added benefit is that it can be used on a PC as well. ie the company file can be transferred between platforms.
Various Opens Source packages also exist, but may not be very well known