On the PCI bus, you have a few options open to you.
First and formost: is the 598 loud enough using the onboard? If not, then a soundcard with an amp is probably needed, which limits you (price wise) to the ASUS Xonar DG.
If the 598's are currently loud enough, then you can probably do without an amp built into the soundcard. Your best options are the ASUS Xonar D1, HT Omega Striker, and if you can find one at the right price, an ASUS Xonar D2.