don't think jumbo frames is supported on this network card. if it was, they definitely would've mentioned it on one of the following links

http://support.dell.com/support/edocs/network/P42481/en/intro.htm

 

Besides, I didn't see an option to configure it in their control suite -

http://support.dell.com/support/edocs/network/P42481/en/bacs.htm

 

 

Do you have the broadcom advanced control suite installed?  As far as I know, if your card does support jumbo frames, you can set it with that.

 

I couldn't find the app listed for your particular model, but this should be the same thing:

 

http://ftp.us.dell.com/network/R116105.EXE