Again, why would you waste money on a replacement soundcard when you can get a good mic pre with digital outs like the ones AR listed for the budget you've given?

Something like a Joe Meek British Channel Strip - http://www.zzounds.com/item--JOESIXQ - which comes with SPDIF and optical outputs would make for a nice upgrade without giving you a bunch of features you don't need - and it will plug right into your M-audio card via a digital coax or TosLink cable.