The Firebox is a mid line interface, good quality for the money, but definitely not what I would consider "high end". It is a nice little box, well built, that supports multiple i/o via firewire and had 2 decent mic pres. The converters are quite good as well.
It does NOT support the 192K sample rate your current card does.

I'd recommend, since all you need is a mic pre for vocals, is to spend your money on a mic pre. Check out the Grace 101 or FMR RNP, both I think are in your price range.