Latency is only an issue when:
Mixing - it is the time it takes for you to hear the result of a change (e.g. you slide a fader). For mixing, latency values in the <80ms range are decent, and any WDM / ASIO soundcard+software combo in just about any computer can do this.
Live Input Processing - when applying effects in realtime to incoming audio. Say you want to add a reverb plugin to your guitar track and monitor it WHILE you're actually playing the guitar track. In this case you want latency as low as possible, but as a rule anything < 10ms is ok. Most decent ASIO/WDM soundcards in semi-fast systems can make this happen to a certain extent...you might not get latency that low if you're running a huge project.
Virtual Instruments - latency in this case is basically the same as with live input processing. It's the amount of time it takes for you to hear a sound after pressing a key on your keyboard (etc). Again, latency as low as possible is good, with <10ms being the rule of thumb. Good soundcard, ASIO/WDM, etc etc.
Latency is not normally an issue when just doing "normal" recording. That's because *most* soundcards support a feature called "Zero Latency Monitoring", which means that the card's outputs are routed to its inputs BEFORE the audio signal even hits the converters. The downside of this is that you don't really hear the exact signal as it's recorded (e.g. you don't hear what the converters do to the sound until you play it back), but the upside is that you can overdub using high latency settings without having to compensate.
Slackmaster 2000