So it may be safer to go for a machine with a separate sound card, although systems are generally adequate these days. It would be nice to have a standard test for sound input quality, so that you could insist on a good performance when you buy a computer. Meanwhile, you should make it clear to the person selling you a computer that you want it for dictation, and that you will take it back if the sound input quality is not good enough. But it does make sense to get the best possible accuracy, not just adequacy. In this case it may be worth paying the extra for a sound card that has been designed for sound input.
Turtle Beach have a particularly good reputation for a clean input signal.
But on the whole we suggest people stick to more standard, simple configurations unless they are particularly keen to deal with computer hassles. For obvious reasons they have to be cheap, and although they may be adequate for sound input quality under favourable conditions, they are often lousy for other reasons. They may be uncomfortable. And the mics often, in consequence, refuse to keep in position. This is important.