My wife has a gateway at work with a soundblaster with onboard amp. It is loud enough to be obnoxious in her little office.
For a given amp, 4 ohm speakers will be louder. That's why car audio is always 4 ohm. It also requires more power from the amp, so if you turn it up, it might sound bad. 8 ohm would be a "safer" purchase, but 4 ohm is probably a better choice.
See if you can dig up the specs on the amp. Maybe it's designed for 4 ohm. If you have two of them you can always run them in series to create an 8 ohm unit.
Bob