over-the-air robust vpn solution for mobile users

Have notebook users that travel to different locations and need to run applications currently installed on their individual notebooks. The applications are tailored for their business and have to access the sbs 2003 shared folders, and, require a drive mapping - F: to one of the shares on the sbs 2003 server..

The sbs 2003 is at headquarters using a basic soho router.  The notebook users use their individual over-the-air carriers like Verizon, At&T, t-mobile, etc on their notebooks to connect via vpn using Logmein Hamachi to the sbs 2003 server but performance is very poor when  the applications access the shares on the sbs 2003.  The notebook carriers bandwidth and headquarters bandwidth both have 3mbits/sec download and 3mbits/sec upload.

Before we start looking into using terminal server or paying for even higher bandwidth speeds, is there a better over-the-air solution for the mobile notebook users that have to use that vpn connection?  

The notebooks are running win 7 pro and home premium.

many thanks