Z_Unit, you're welcome.
NRen25k, good point, but while this is technically true, in my opinion and experience with actual hardware, it will be even more likely to go the opposite way, though, when all things are considered. I admit that this does have a lot to do with how electromagnetically noisy your computer is, however. That is one reason I buy quality motherboards and components. I was very concerned about EM radiation back in the 486 days. But even with cheapo motherboards, I have found that actual componentry has far, far more to do with sound quality than physically getting your analog mic cable away from the computer. Monitors are more noisy, and these 'stick' mics are usually in front of monitors, but even so, nowadays monitors are good enough that sticking a mic in front of a CRT is usually not going to cause problems, at least if it's not 'facing' the tube from close range. LCD's are far more healthy... seriously.
Now, with extremely sensitive mics like large-diaphragm condensers doing ambient/sensitive sound recording, yes I wouldn't want to be plugging directly into the comp (hence my experience with USB recording), but it still probably wouldn't cause a problem. That'd be one nice-sounding signal still. But with the quality of these "stick" type microphones, the quality of the actual mics, how far away they are from the mouth, and the type of ADC and the resolution it runs at are all going to far outweigh the fact that the analog mic signal is not running directly into the comp.
In my experience, the actual quality of the sound card, mic, and mic placement are by far the three most important factors in sound quality, rivaling cord quality, mobo type, and even being finicky with recording resolution (a high-res recording of a crappy mic will still be a crappy recording). I have progs which visibly display the noise floor, and I can say with satisfaction that w/my comps, any gain in quality from getting the ADC away from the comp is far overshadowed by the quality of the componentry (sound card, in this case) itself. I.e. I have not had interference issues.
In fact, the one time I did have interference issues was actually using a USB audio interface. I was receiving a buzz through it by way of the computer. You see, even though that USB cable is only supposed to be transmitting digital signals, in reality there is still a conduction between the computer itself all the way to the mic. I no longer remember how I resolved the buzz, but there was some analog buzz coming from somewhere inside the computer creating the buzz which traveled all the way up the (supposedly pure digital) USB cable, out the headphones & audio out, into my stereo. I don't know if it would have shown up in recoding, but that analog interference (buzz) definitely came through into my interface, and others experienced a similar thing. I realized then that USB audio devices aren't so "separated" from the noisy environment of the computer as is normally assumed. It's still connected to the computer through metal/conduction, no matter if someone says it's theoretically impossible.
In the 'stick' mic's case, the 'sound card' is located in the housing itself. For this user's case (voice recog) I think mic placement is going to be the most important factor beyond anything else, followed by actual mic quality. USB vs sound card is irrelevant if all other things were equal (i.e. mic quality), but of course they're variables.
> i go with USB which should have line
> noise canceling instead of analog one.
I'm not sure that you understand what "noise cancelling" means here. This is another term used to manipulate people like "digital headphones" (though not as bad). Note that they do not explain what "noise cancelling" means here. It probably means that "the mic is not omnidirectional", and that "the mic is pointed at where we think your mouth will be, with maybe plastic behind it". This is nothing glamorous and just about any type of computer mic application, headset or not, should obviously incorporate this without need to state it. People are misled to believe that there is some kind of electronic or "active" or even "digital" noise cancelling here, especially with "digital" USB mics, and that is not the case. If it had "active" noise cancelling, a somewhat exotic treatment which I'm not even convinced is available for mics (it's available for headphones), trust me they would say it, and the device would be MUCH more expensive. So, the way they are describing "noise cancelling" here, any solution should have those aspects. (I suppose "line" noise cancelling suggests the "reverse Faraday" concept NRen2k5 addressed earlier.)
In my opinion, mic placement will be far more important, because the farther it is from your mouth, the weaker the signal will be, the more the signal will have to be amp'd, and the more ambient noises it will pick up ("noise cancelling" or not). A headset mic stays at the same location and is more controllable. I don't know your setup, but however it is, you won't be able to move around a lot and have the comp recognize your speech (of course, tied to the leash of a headset doesn't help in moving around much, either). Hopefully, you won't have to 'lean in' to the mic to get successful results. Voice recog progs are more fallible than human ears, which is the main reason I'm concerned. Just make sure you have a good return policy, and do update the group to let us know what your result was so we can learn. Hopefully everything will be okay, obviously.
BTW, a good way to measure EMR is to take a handheld AM radio, tune it to a clear station, and scan around your comp, monitor and components listening for interference. Another way is to take a mic itself, if you have it plugged into a preamp feeding into headphones, and "listen" to your componentry. My experience is that unattached periphs like wireless mice, CRT monitors, and wireless broadband routers (wireless devices in general) and even cable modems produce more EMI than modern decent computers. O/c, this varies case by case. But I can tell you that a wireless mouse is the noisiest damned piece of hardware I have seen since the death-ray CRT which came with my first 486.
A lot to chew on, but I'm an "audio freak" much the same way this is a home for "CD freaks". I've enjoyed the discussion.