how can i know i'm buying the right memory kit?

hi there.
i just got a Mac Pro Quad-Core Intel Xeon dual 2.8 GHz with 2GB of memory.

i found an 8GB ( 2 x 4GB ) DDR2 PC2-5300 ECC Fully Buffered Memory Kit for
Apple Mac Pro Quad & 8-Core online for about $700

i've also found DDR2 PC2-6400 for about $50 more.

how can i be sure i'm buying the right memory for my computer.
they seem to be very model-specific.

do these prices sound reasonable?
i'd rather buy the 4GB chips to have more room to expand in the future.

i appreciate your help.

kiz

The System Profiler utility will tell you whats already inside your mac so at least you can compare the memory types.

The Crucial Website has a system profiler that will guarantee to find compatible ram

You need the PC6400 RAM. The PC5300 is for the older Mac Pros. Try macsales.com. I suspect their prices are much more reasonable and they clearly state which speed is for which model.

Per mrplow, the Crucial Web Site has a "Memory Advisor" that will lead you to the right memory. In this case, you want the Mac Pro, Xeon 5400 4 core type, which leads you to this page. From this you can see that what you want is DDR2-6400.

I have the same generation of Mac Pro and bought memory upgrades from Crucial. They work great, so I can confirm that this is the right memory for the machine.

many thanks.
it was $50 cheaper at macsales.com so it's on its way now.
bought 500GB drive to boot.
can't wait.
thanks.