You can rest easy, Macs have not had problems with viruses...when people have had problems they have been related to falling for scams and malware that they mistakenly thought they needed.
Avoid anti-virus software, especially commercial products like Norton which has recently caused some serious problems for users on here.
The extended warranty from AppleCare is very good, especially if you don't have the resources to just go out and buy new equipment. They are very good about helping with problems should they occur.
Adding my two cents worth:
At this time there have been no confirmed Mac OS X viruses (subject to the semantic debate about whether the old iChat exploit, now long since blocked, was properly classified as a virus or a trojan horse, and not counting the Word macro virus that can affect Office v.X or non-updated copies of Office 2004), very few trojans and no adware or spyware. I therefore do not feel that antivirus or antispyware software is necessary at this time as long as users are careful about what they download from the Internet or accept as attachments via email.
The most common exploit is a recent outbreak of "scareware" that people are encountering while web surfing. This "scareware" pops up and claims that your system is infected with viruses, prompts you to download their "scanner", then tells you that you need to purchase their paid software to remove them. These fake applications, appearing by many names including Mac Defender, Apple Security Center, Apple Web Security, and many others, do nothing at all other than turn off the fake warnings once they have your money. DO NOT fall for these.
Security precautions for Mac OS X mostly are common sense: don't download and run files from sites you don't know, don't fall for any "you have viruses" messages, don't double-click on attachments in emails you aren't expecting and/or from people you don't know, and make sure you password your system with a non-trivial password and keep that password secure. In addition, don't leave your system available in a public place and unattended where someone unauthorized could use it and potentially install malware (such a policy is also a precaution against theft).
It is possible for a Mac user to inadvertently forward a Windows virus to a PC user though you'd have to do it manually so the above precautions would probably prevent such virus-spreading, and said Windows virus cannot infect the Mac.
If you do decide you wish to run antivirus software, I think that the donationware clamXav should be more than enough. iAntivirus has also been recommended by others, though I have no personal experience with it.
If you are running Windows on your system, either via BootCamp, Parallels Workstation or another solution, then that copy of Windows is subject to all the myriad exploits common to the Windows world, so you need to take full precautions, including running both antivirus and antispyware software.
And if I could add one thing to all of this. If you ignore all of the advice from long time users of the Mac (I also do not use an anti-virus), do not, and I repeat DO NOT UNDER ANY CIRCUMSTANCE, download, buy, install, or use MacKeeper, which sells itself as an anti-virus, but if you read anything about it here or from Mac tech writers, it should be classified as malware.
To add to the excellent advice already given, I would refer you to my Mac Malware Guide. It includes a lot of extra details, along with descriptions of all the Mac malware that I'm currently aware of.
As to the extended warranty, note that that will not cover accidental damage, so it really isn't more important for portables than for desktop machines.
As to the extended warranty, note that that will not cover accidental damage,
It's covered under your Homeowners/Rental police as well as your Auto Policy (Property Damage) in some cases. Then there is also:
If your homeowners, rental insurance is not sufficient, you do not have any insurance and/or your Apple & AppleCare warranties have expired you may wish to consider Safeware Insurance. Coverage includes the following: Accidential damages Dropped laptops Liquid spills ~~Stolen comp~~ ~~Natural disasteres strike~~ Peripherals Software Personal date against theft ~~power incidents~~