As a new Mac user, I asked the initial questions you did: Do I need protection? How much? And is a free program sufficient?
I can tell you this much. I, along with Apple now, suggest Anti-malware software. I originally had the free Sophos Antivirus program and was happy with everything except the lack of scheduled full scans; you have to remember to do them manually. Since I'd heard Macs don't have virus problems, I considered this sufficient. Then I found my Yahoo email sending spam to my Contact list by way of a Trojan after having my MacBook Pro for only 3 months. I ran a full scan on my Sophos and it did find the Trojan and instruct me to remove it but I would never have caught it if a contact of mine hadn't brought the Trojan to my notice first.
So I did more research on Mac Anti-malware programs. I found that ClamXAV does have scheduled scans but reviews suggested it took longer to catch new risks than Sophos. Since I share a network and many files with other PCs, I've decided it might be worth the peace of mind to get one of the paid Anti-malware programs.
Of the paid programs that protect against both Mac and Windows threats:
Norton's program is well-rated but, as a previous customer who has learned that their software takes up lots of space and is near impossible to remove from your machine, I won't go there again. So I'm likely to go with MacScan or Kaspersky and trade $30 or $40 for some peace of mind.
I suppose this is a round about way of saying: Anti-malware is now necessary as malware grows with Mac's growing popularity. Get the free ones if you aren't worried about the amount of contact you have with PCs or feel your firewall is sufficient. And if, like me, you just want to be sure then maybe it's worth a little cash to feel protected. It really does depend on your circumstances.
Good luck!
-Lauren