Your Mac didn't ship with a sound card -- Apple computers are wired for basic sound already. If you had a physical sound card, you'd be able to open up your computer and see it in one of the PCI slots. You'd also see the outputs on the back of your computer, in one of the slotted positions.
A quick way to find out which cards *are* installed is to go your Apple menu, click on About This Mac, and select the tab marked Devices and Volumes. You'll be able to see the card type, card name, vendor ID, ROM number and other information.
Yes, of course you'll see an improvement with the Sonica card (or virtually any other sound card, for that matter). I'm not being sarcastic, blessingx. I just want you to understand there's going to be a difference because nearly any compatible sound card will be better than none. The outputs will be cleaner at the very least.
About your choices:
The Audiophile 2496 and Revolution are both said to work quite well with Macs running OSX. The latest driver for the Revolution even matches the latest OS release: 10.2.5. I haven't seen reports on the Sonica Theater with Macs, so I'm a wee bit leery. What I can tell you is that older G4s -- the ones that have conflicts in sleep mode with certain PCI cards -- are said to run the Revolution card without any issues in sleep mode whatsoever. A quick search in the Mac Discussion Forums will bear this out.
For the future, I'm keeping an eye on Logic Audio and its attendant hardware, since Apple bought the company. For now, I have a Mark of the Unicorn PCI-424 card to go with my hardware recording interface.