Hazel,
What kind of sampling do you intend to do? Do you need something that can playback whatever you sample? Then, maybe it's not a soundcard you need but maybe sampling software.
For example, I learned about sample manipulation and soundfonts using a soundblaster. Once I converted a wave to a soundfont, it was fairly easy to manipulate but sound mangling tools were missing in most soundfont tools available. The best tool, for me, to manipulate and mangle a sound was Kontakt. I could drag a soundfont into Kontakt and just play it. I could layer sounds easily. I could mix and match different sound formats if I wanted to. Kontakt doesn't need a soundblaster, it will work with whatever soundcard you have. You're much better off using your RME to sample anyway.
Recently, I've been using my EmulatorX software and it's filter algorithms are fabulous. The only caveat is that you need Emu hardware to run the software. IMO, all you need is good software since you already have the hardware figured out and the RME's converters are much better than anything you'll find in a 'blaster.