What version of Softphone are you using? We have a dozen or so deployed across a variety of remote sites with mixed results. Typically I recommend a 4meg internet connection as the minimum as it covers the download/upload requierements and then some. Keep in mind that you will be losing QoS for the most part, and will likely encounter issues with call quality and disconnects from time to time.
I have seen Microsoft Outlook cause some issues with the softphones, specifically when opening attachments, but it tends to clean up rather quickly after the attachment is opened. I have had good luck changing the softphone process priority to high on some older machines. If i can think of anything else I will post up.
I would imagine GHTROUT and bigindian can offer some great advice as well, their my best source on this site.