The mac operating system is based on Unix, and as such it is harder to exploit than Windows. Add to that the fact that there are far fewer mac machines around than PCs running Windows, and you can see why Macs are not such a tempting target.
That said, malware for Macs has increased dramatically of late - it increased by over 250% last year, and new threats already identified in the first three months of this year indicate a similar rate of development.
You should definitely get some good antivirus software on your new machine.
According to our sister site (Macworld) this would be an excellent choice.