I have just recently switched to Apple and purchased a MacBook Pro for home use. I am somewhat lost concerning what virus protection is "the best" to run. As with PC's there seems to be several offerings. I picked a "well known" version with my PC; however, a virus(worm,spyware...something) was still able to defeat the anti-virus and mess up my OS.
I've used Symantec Anti-Virus but I have not installed any for my MBP. I am not sure if Symantec Anti-Virus is available for the Mac Leopard yet. I will say that Windows may have preconditioned you though. Open your mind to the force Luke, let yourself go. Many Mac users don't even install Virus protection. If you really want it, fair enough, but I don't think there is any really big hurry to buy one and install it.
The last time I used a virus scanner on Mac OS was about 12 years ago. It only produced one false warning. The threat of viruses on OS X is much, much lower than in Windows, due to the fact that there's no real working virus for OS X yet, as far as I know. There are some concept 'viruses', but they all require user interaction to install, and with the new security features in Leopard it's very likely that you will notice when something fishy is going on (e.g. your Mac warns that you're opening a program when you have double-clicked something that was supposed to be an image).
I think that at the current time, it is more useful to maintain good security practices (enable your firewall, don't download and run any 'dubious' programs, check software update regularly and install security updates immediately,) than to install a virus scanner, which will only stress your system unnecessarily.
There are some situations where it's useful to install a virus scanner in OS X, though. First, if you use Boot Camp or Parallels (but then again, it may be more useful to install a PC virus scanner in your Windows partition). Second, to detect PC viruses, so you can tell if someone's PC is infected and prevent the virus from spreading to other PCs.