what is your current soundcard?
It really depends on how much you want to spend. If you only need one input, the audiophilie 192 may be a good choice. If you want to spend more there's RME and MOTU. If you really wanna spend more, there's lynx (i have heard awsome things from these cards).
If you wanna go even a further step up, i think you can get standalone A/D converters. Some, such apox, are really high quality and cost a lot. I don' t know much about them as they are way out of my price range. To use these, i would think that you would need equally nice preamps, mics etc.