Audio interfaces that would do the trick start at around $150, and a larger number are available for around $199.
You will want to make sure which ever one you get has drivers for Windows 7, and realistically Windows 8.
There are some with USB connections, some with FireWire, and PCI/PCIe.
I happen to use an interface with a USB connection - please note that many USB equipped audio interfaces do not play nicely in USB 3.0 ports, so just make sure you use a USB 2.0 port.
Just about everybody in the forum can help get you up and going if you have any interface configuration issues, so don't worry.
Audio Interfaces move the process of analog/digital conversion away from the CPU, in addition to having pre-amps and inputs/outputs and such. This will make your CPU work much less hard to do audio processing in Sonar.
Hope that helps,
Bob Bone