To say Macs aren't "as susceptible" to viruses as PCs is a gross understatement. There are no viruses for Mac OS X. None at all. Period.
My advice is not to bother with anti-virus software. It's an unnecessary cost and hassle. Some virus scanners (*cough* Virex and Symantec) are also known to cause stability issues.
If a single virus ever shows up in the wild, you'll be sure to hear about it. Until then, spend that money on something important, or buy some software that will actually be useful. Panic (
www.panic.com) and Omni make some great products for the Mac.