Your level of risk with Snow Leopard varies depending on the threats.
Viruses: Malware that propagates automatically and spreads from computer to computer. This is what most people think of when they think of security issues. It's unlikely that you will get a real virus on Snow Leopard. None exist in the wild (at the moment) as far as I know, though it's possible that there could be some in the future.
Trojans: This is the classic nasty email attachment or innocent looking download that takes over your machine. It's most likely avenue of attack and the most likely to succeed, but also the easiest to defend against. Don't click on suspicious email links or attachments and don't download software from shady sources and you will be safe from trojans. It doesn't matter what OS you run; smart practices will keep trojans from being an issue.
Browser exploits: This is a pretty common attack that is usually moderately successful. It would happen if you visited a malicious site, or if there was a malicious script running on an otherwise benign website. There have been cases where malware authors have paid to have ads on legitimate sites and installed their malware instead. Many of these exploits are Flash and Java based, and not getting security updates for Java and Flash would be a big deal. In Firefox, you can install NoScript to block all scripts automatically. This will prevent malicious scripts from exploiting your machine, but it will also prevent normal scripts from running as well and may break many websites. You will have to manually allow the scripts you need, which isn't hard, but can be a little irritating.
Individual hackers: This might be the most dangerous attacker, but the least likely. Most of the time, this occurs on public Wifi networks where a hacker scans the ports of the machines on that network and attempts to run exploits against running services on them. Since you are asking about a desktop, I can't imagine this being an issue, unless someone got access to your home network throgh your router. Router security is not too impressive in my opinion, but successful attacks against them are rare. A much more likely problem would be someone guessing your wifi password and getting himself on your network, then port scanning your iMac and attempting to exploit vulnerable services on it. In that case, a lack of updates to your OS would be a problem. However, the chances of this actually occurring are low and you can mitigate the risks by using long, random passphrases and WPA2.
