you need to find dual opamps in the dip-8 package (cdip and pdip should both work, being either c-eramic or p-lastic). the dip-8 duals are all pin compatible, but as someone else has already mentioned, there are some circuit specifics that will determine whether they work well or poorly in your card, and the manu is not likely to give you a schematic to help with this.
unless you are very good at soldering, i would not try to solder soic (surface mount) chips to an adaptor, but if some are available pre-made, you may want to try them.
also, opamps will give you very different sound, as you swap from one to another, and it is hard to find reliable sonic impressions on the web, weeding thru system-specifics and subjective tastes (as has already been mentioned).
i would recomend doing the unthinkable, and letting the card burn in with many hours of play and decide what improvements or changes you want, and then trying to find opamps which will give you those changes.
i've been doing this with both a prodigy hd-2 and a xonar essence, and it is worth doing, but is time-consuming and takes some warm-up and careful listening.
mouser and digikey are great resources, and you can at least compile a llist of opamps that will work as direct plug-ins to your card, and go from there.
auzentech has some they sell, and give brief descriptions of their sound as well.
do your homework, be honest with yourself, and you will get better sound without buying every opamp in creation.