For the internet radio side of things pretty much anything will do. Even the lowest spec new system should be able to handle decompressing a Real Audio (or similar) stream and playing it back. There's not even that much point in getting a fantastic sound card since the quality of the stream isn't that great in the first place.
Real.com list the following...
Minimum System Requirements for RealOne Player and RealOne Player Plus*
- 233MHz Intel Pentium II processor
- 64MB of RAM
- 28.8Kpbs modem (audio only)
- 16-bit sound card and speakers
- 65,000-color video display card set to display at 800x600 (video)
- Windows 98, Windows 2000, Windows ME, Windows NT 4.0 with Service Pack 4 or later, Windows XP
- IE 5.0 or later
Recommended
- 300 MHz Intel Pentium III processor or greater (supports simultaneous record/playback features)
- 128MB of RAM
- 56Kpbs modem
- Full Duplex sound card and speakers
- 65,000-color video display card set to display at 800x600 or higher (video)
- Windows 98, Windows 2000, Windows ME, Windows NT 4.0 with Service Pack 4 or later, Windows XP
- IE 5.0 or later