With an iMac, there is no internal upgrade, so instead of looking at "sound cards" you should focus on a DAC (which is an "audiophile" term for external sound card). ;)
Which iMac do you have?
If it's an Intel based Mac, it most likely has an optical audio output. If this is the case, I find the optical output to be extremely good, especially when using a nice glass mini toslink cable.
If it doesn't have the optical output, then USB is a great option. You can get USB DACs or a USB to SPDIF adaptor. I recommend the HagUSB.
As for DACs....well....that's a complex issue. They range in price and features, and of course quaity and character of sound. It all depends on you wants, needs and budget.
Which iMac do you have?
If it's an Intel based Mac, it most likely has an optical audio output. If this is the case, I find the optical output to be extremely good, especially when using a nice glass mini toslink cable.
If it doesn't have the optical output, then USB is a great option. You can get USB DACs or a USB to SPDIF adaptor. I recommend the HagUSB.
As for DACs....well....that's a complex issue. They range in price and features, and of course quaity and character of sound. It all depends on you wants, needs and budget.