What is the key parameter for a sound card, if I want to record at home using Guitar rig? I've heard of 'low latency' and 'no latency', but I can't find this parameter in sound card descriptions. What should I look for to get clear sound with the lowest possible delay?
"Clear sound" is a property of the digital-to-audio converter ("DAC") and is mostly subjective. Brand reputation is helpful here, as are reviews.
Latency is measured in milliseconds. It is a combination of a number of factors - the sound card and drivers you're using, the hardware overhead (USB or firewire can add a couple of ms) the protocol used (ASIO, WDM, Directx, Core Audio, etc.) and the available resources your computer has, like processor and memory.
For low-latency operation you'll want to be using one of the protocols designed for this. On Windows this is ASIO, on OSX this is Core Audio. I don't know what the standard is for Linux. Most hardware aimed specifically at recording music is compatible with at least one of these.
Generally Firewire is slightly lower latency than USB from what I've seen, but I don't have any objective reference to back that claim. I use a USB soundcard myself.
In my experience, not all soundcards are created equal as far as latency. I don't know why this is. When you've picked one, see what you can learn about it before you buy it - read reviews, ask on forums, ask friends. Make sure it fits all of your criteria.
Unfortunately there doesn't seem to be any well-advertised parameter for latency.
To make a recording without runnig in to any problems like latency and clips (unwanted sounds) your computer configration is also very important as your sound card. Real time recording needs lot of processing power. Which means you can start with core 2 duo 2.0 GHZ processor and 2 gb of ram and 7200 rpm harddisc. You can also use lower configration computers, but in my experience since 8-9 years of studio recording this configration will work well.
If you can write down of your studio setup, computer configration, sound card and gears i can give you the parameters for ideal guitar, vocal and the other instrument recording.
When you read "no latency" it is actually referring to a monitoring feature where the analog inputs can be mapped to the analog outputs directly without going into your computer. If you are simply recording, this is fine. If you are playing with amp modeling or any other effects on your PC, then this feature doesn't matter to you.
All the cards built for this purpose will advertise low latency, so you can ignore this point. What you're looking for is if it supports ASIO (Windows) or Core Audio (OSX).
The reason you don't see latency specs is that it is entirely dependent on your system. As Warrior Bob pointed out, drivers, interface, etc. have a lot to do with it. Some folks will tell you that you have to use Firewire to achieve a low latency signal chain... this is false, and at least outdated. Either Firewire or USB will be just fine. I use USB, as every computer has it, and the computers with Firewire interfaces often have driver issues. (That however is a religious war for another topic...)
I'd say that since you are recording guitar, make sure it has a guitar input as well as other inputs as needed. Even if for now you are using direct outs for recording, you might want to try amp modeling some day, and that guitar input will come in handy.
That's really all there is to it, as far as specs go. From there, read reviews online. If a lot of people have trouble with a particular unit, stay away from it.
If you are looking for amp modeling, check out some of the stuff from Line 6. http://line6.com/toneportux8/
If you don't have a budget for sound card you can use your on board sound card for recording. But the result will be very poor sound quality. All you can do is download the asio for all driver for free. This driver will fool your recording program and shows your onboard soundcard like a professionel sound card.
With this dirver you can use latency down to 256 samples. If thre are any new begginers for computer recording with no budget they can tyr it out.
If you want to use Guitar Rig or Amplitube for recording there is couple of sound cards you can get from the market. M-Audio products like (M-Audio Fast track Pro) is a good one to start with. You can easly adjust the latency settings down to 128, and it has a direct monitoring for easy recording. It has enough inputs for guitar, mic, and for another sound source. 48V fantom power included. No need to use any electric adapter, it feeds from the usb.
IK Multimedia is also a good brand. Im using Stealth Pedal for live on the stage and for recording too. It has 2 inputs for guitar, bass guitar or stereo input for keyboard, mono or stereo outputs, externel pedal conncetions for volume and switch pedals. Looks like very cool traditional wah or volume pedal. Pedal comes with couple of programs for your need. For more info check the link below.