Any Ram more than 1 stick needs to be paired.  That is why the Ejectors are white and black or some other contrasting colors.  Depending on the FSB of your CPU you may need PC2 8500 ram.  For many of the newer machines anything less than PC2 6400 is TOO SLOW.  

PC2-3200 = 0400 MHz  clock

PC2-4200 = 0533 MHz  clock  (Intel SL8ZY Pentium 4 517 2.93GHz  533MHz)

PC2-5300 = 0667 MHz  clock

PC2-6400 = 0800 MHz  clock  ( Intel Core 2 Duo Processor E4400 2.0 GHz)

PC2-8500 = 1066 MHz  clock  (Intel Core 2 Duo E6300 1.86 GHz )

How much memory your Windows OS will recognize depends on which version of Windows you are running. 32-bit versions of Windows will see (and utilize) only 3GB or 3.5GB. To utilize more memory, install a 64-bit version of your OS. More information about OS memory maximums can be found here.

This also means you don't actually Get 4 or 8 gigs because the onboard INTEL Video steals some ram that you never get back.

PC2 8500 Ram is for cpus that use 1066mhz fsb

PC2 6400 Ram works for cpus that use 800Mhz Fsb.

If you always use PC2 8500 Ram it will Always work with any 755.

2GB, 240-pin DIMM, DDR2 PC2-8500 memory module

Crucial Part Number: CT751693

   Module Size: 2GB

   Package: 240-pin DIMM

   Feature: DDR2 PC2-8500

   Specs: DDR2 PC2-8500 • CL=7 • Unbuffered • NON-ECC • DDR2-1066 • 1.8V • 256Meg x 64 •

   What does this mean?

Guaranteed-compatible with ALL Dell OptiPlex 755 Series (Desktop Mini-Tower and Small Form Factor)

www.crucial.com/.../mpartspecs.aspx

Thanks for the info!

I looked on the bios and on the Processor Info screen it says:

Processor Type = E8500 @ 3.1 GHz

Processor Bus Speed = 133MHz

On the Memory Info screen it says:

Memory Speed = 800 MHz

So would it make a difference if i use PC2-8500 or PC2-6400?

Thanks Again,

Steven