Motherboards may have different revisions, and depending on the rev. the RAM specs may be different. In your case, go to: http://accessories.us.dell.com/sna/memconfig.aspx?c=us&l=en&s=bsd&cs=04
Then enter the service tag number. Based on this tag number the RAM specs will be listed. Make a note of these specs. and see whether there is any suitable RAM with a higher clock speed and/or tighter timings. Verify from Dell. Now shop at your favorite store for best price and delivery.