Vegas does not use your graphics card for playback so buying a new one won’t help. Vegas uses pure CPU power and a 2.13Ghz is at the low end of the scale. If these files are highly compressed that could contribute to the problem.
Your two options are to get a more powerful computer or render the video to a format that Vegas can preview faster. If these are SD quality files, rendering them to DV AVI would be the fastest format for playback.
If the customer brings the file with them and expects to see it immediately (i.e., there is no time to transcode) then your only option is to get a more powerful PC. 🙁
~jr
http://www.johnrofrano.com
http://www.vasst.com