Actually amp quality has as much to do with sound quality as the speakers. An abundance of RMS power will give you better sound than straining a cheap or low powered amp like what comes in most computer speakers (like the 7watt rms sets everyone has).
Now the logitech z560 has an abundance of highquality power, and that in my opinion has more importance than big or expensive speakers.
Start with quality highpowered amp then go for speakers.
In fact, under powering speakers does more harm than over powering speakers.
As far as your original question, thats an easy one.
The speakers and amp are going to have the biggest impact. Now if you had an incredibly terrible output that would be a moot point but AC97 is "good enough".
Good enough to where I'd rather have that and Martin Logan speakers and amp hooked up to AC97, rather than an Audigy and Altec Lansings.
Spend you money on a quality system, if you have a 2.1 AC97 setup get a real 2.1 home stereo amp and speakers (not computer speakers), you'll get alot more volume, deeper bass ect. In fact no computer speakers offer over a 8inch woofer.
If you have a 4.1 setup you might as well save some money and get a "computer speaker" setup, it will likely have a shielded sub and will work well with 4point surround EAX games.
In that case, get a Audigy and the Logitech Z560s.
"dude your getting a dell", is that kid trying to say he wants to stick his 'dell' in you?