I cant say it better than my sonar brothers have above, like chregg said,what are your needs?
a lot has changed since sonar 4(im sure you are seeing :)
soundcards have become more advanced, operating systems have changed as well.
with that comes more taxing needs, but even if you want to use sonar as a basic entry level recording system,
you need a half decent soundcard.
most of us these days, but not all of us use what we will call for explaining purposes... a break away box.
an audio interface (soundcard) that uses an usb cable to plug in to your computer.
like this:
http://www.rolandus.com/products/details/1089 now,do you use midi?
do you use 1/4 guitar input jacks?
these are questions you should ask yourself before buying a soundcard,
ask yourself as well, is this device been tested and have up to date drivers.
the worst feeling your going to ever have is buying a soundcard, bringing it home and realizing
that they haven't made a driver for your operating system.
so plan good and hard,ask questions like your doing now.
if your in a bind and need a quick fix.
try using asio4all:
http://www.asio4all.com/ this is basically the same as an audiointerface/soundcard but with out the inputs.
this is a temporary solution to a problem that will eventually need a permanent solution.
now try to sit back and take all this in with a cup of coffee :)
by the way graham,
Welcome to the forum buddy :)