I agree that speakers will make a bigger difference in grand scheme of things but if you do already have even decent very entry level speakers, you'll hear the difference a decent card can make.

Obviously some on board soundcards are utter turd and should be disabled in BIOS before ever allowing a signal to be tranduced but the newer mac ones aren't that bad so in terms of priority, the speaker upgrade should come first.

There are several improvements a soundcard will offer:

Latency - this one is obvious but it does also affect genera timing of you project especially if using anything external, with samples and midi.

Performance - especially in the case of PC's, a soundcard with decent drivers will take the load off you CPU which ultimately lets you do more etc.

I/O - you can connect more, duh.

Quality - going from the mac built in sound to and Audiofire 4 was a serious and noticeable upgrade in terms of clarity. Wierdly, I heard more problems with previous projects that I thought were OK and a lfair amount of them even had to do with timing issues so I feel quality issues are affected on many levels by the quality of your soundcard.

Basically, if you have OK monitors then go for a better soundcard but if you're monitoring though your $100 hifi then really think about monitors first before buying a soundcard.