Are you really looking into reference monitors, or just better computer speakers?
Usable passive reference monitors can be had for $200-300, but you will of course require a reference amplifier which will run you in the $200-400 range. Another option is to look into active monitors which are basically monitors with built in amplifiers. They will typically run just about as much as seperate components of the same quality, but are simply easier to purchase and use (and arguable they might sound better since the amp is build explicitly for the monitors, but that's another topic).
Then you will need some way of controlling volume to your monitoring system. A small mixer or line preamp would work.
The type of soundcard you have isn't really much of an issue.
Slackmaster 2000