Here is a link to the Nvidia web site where the technical specs of the 8600 GT and the 8600 GTS are listed.  There are some 8600 GT cards that come factory overclocked to approach the clock speeds of the 8600 GTS but I think you would get the common garden variety of 8600 GT from Dell.  In terms of retail card prices, the 8600 GTS easily can cost $50 more than the 8600 GT so going with the 8600 GTS, if you want a little extra power, wouldn't be a ripoff.  http://www.nvidia.com/page/geforce8.html

Performance is the difference, flip thru the pages here;

 

http://www.tomshardware.com/2007/04/17/geforce_8600/page5.html