It's entirely up to you but I personally have found that on board audio is actually very good. I must admit that my five year old creative intrigue speakers are dated and probably do bottleneck the X-fi Forte sound card somewhat, but even so they are still good quality speakers. I find the sound very good indeed. However, when I upgraded my sound card I was left a little disappointed. In fact very recently I took it out and relied on my on board audio for a few weeks, I honestly couldn't tell much difference.
So the only conclusion I can draw is that a new set of speakers in my case would probably be the solution and would blow me away, even though what I already have is imo very good.