I have an ATI 6250 card w/1GB running 3 HD monitors. I think I paid $50 for the card.
This is more than enough for Sonar if you are not doing music for video.
Drawing the Sonar screen even on 3 monitors is a trivial task for even a basic video card.
I used the same video setup in an 8 yr old Core2 Duo 3Mhz E8400 for most of the Sonar X series.
The following are some things we need to know before trying to answer.
Which audio devices are you using?
Have you disabled any audio devices (e.g. HDMI) associated with the GT240.
Are your drivers current for both audio and video?
Are you using ASIO mode for audio?
What are your buffer settings?
How much RAM do you have?
Are you streaming sample data from Kontakt or other sample players?
If so are the samples on a separate HDD?
Are you recording to an external HDD?