Does your opamp on the sound card have a socket? If not, forget it. You are obviously not ready for SMD soldering and parts removal. As for changing the resistors to adjust gain, I doubt seriously there are any resistors on the sound card that are not SMD - forget it.
If your opamp has a socket, (DIP-8), then yes, you can change the opamp. (You can change it even if its SMD, but it takes a pro or experienced DIY-er.) The thing you need to do, though, is look at the opamp that's there. What kind is it? Read the label on the opamp and then look up the datasheet. The datasheet for the opamp will tell you the operating conditions for the opamp. From that, you can start to pick other opamps that fit into the same voltage/current category. That will at least narrow down the search for what will work. As DutchGFX says, opamps are designed to work with certain voltage and current supplies. If you pick an opamp that's not compatible with your soundcard's voltage and current supply, it won't work and may burn up the opamp or something else on the soundcard.
This isn't as difficult as it sounds - most opamps work within acceptable limits of certain design classes. For instance, there are opamps that work with 24VDC, 12VDC or even 5VDC. There are few that stray inbetween those numbers, but it's important to know which.
Also, opamps come in two stereo configurations: single channel per opamp, or two channels within the same opamp. The same opamp series may be available in both. It could be DIP-8 or DIP-16. These are all things you need to investigate. Unless you can find some specific articles online where people have changed out the opamp on your specific sound card, you have a lot work and research to do - much more than you can get answered here.
You could try the DIY section of Head-Fi, but most people over there do not mess with soundcards. I'm not sure you realized how difficult a question you were asking, especially for this section of Head-Fi.
Anyway, good luck.