I have Asus P5B-E Plus, should i buy a sound card ?

My Creative Audigy cards work terrible with the Vista the drivers Creative currently supplys . If you are installing a cheap Creative card on XP OS you should be fine. Try downloading the latest sound drivers for your MB from the ASUS website. I have several ASUS boards that have good enough onboard sound to avoid Creative and their pathetically poor Vista drivers.

It's mostly a Vista issue. They changed how you manage the sound outputs on a lot of sound devices. It works, but you don't have all the available options you might have on XP.

would there be a big difference (assuming i would be using win xp) ?

Buy an X-FI Xtreme music or better, otherwise don't waste your time. Unless you specifically need the input or output of these cards, or are really an audiophile who has to have the crispest of sound from your game ( I have one, and it does sound amazing, I love it, but that's just me) your onboard sound will suit you fine.

Either way, it is a last last last priority. It's a nice upgrade when the 3d sound really works out like in Oblivion or BF2, but it's by far not even something to consider unless absolutely everything else is taken care of.
I'd place it behind even a good set of speakers. Speakers can make or break any sound card.

Performance wise there is a slight smoothness upgrade I've noticed relative to using the more complicated sound options in games. When in doubt the rule of thumb is dedicated hardware is always better than software codecs at the very least in performance. Modern CPUs keep the usual performance at an even keel but you'll find the quality, especially 3d quality is better with the luxury of a dedicated sound card.

That being said, unless you were going full out with an x-fi or other specific card to your specific audiophile need, don't bother.

well.. i have an altec lansing 5.1 speakers... dont know the model though... but its really good...

When and if you ever want to get into low latency recording sound interfaces. Look at the RME website. RME interfaces have great support and they work as expected.

Chuck