What you can physically fit and the space available is not the same thing, Crucial is an excellent site but it might say that you have the capacity to fit x megs but whether you have the empty slots is different (see the last paragraph), and you will not really know untill you open the case and look.

The actual fitting is simple enough if you can reach the slots without dismantling the whole machine!

Supposing you can clearly see and reach the slots, with the machine off, and the power cord removed - yes removed! Gently press on the white clips at either end of the fitted memory it will pop up, lift it out without touching the gold terminals, notice the slot in the bottom edge.
Now, and without altering the orientation, place it back in the slot lining up the slot to the peg in the receiver. Now with a little pressure you can clip the memory module back in the slot and the tags will pop back into the locked position.
Okay you have done it, its as easy as that. So when the new stuff arrives you will know how to fit the new modules.

Your new modules could have twice the value of the old ones, so if your board can handle say 1 GIG of memory in two slots, thats a max of 512 megs in each, you might have 2 x 256 megs fitted and there are no empty slots, so pop out the 2 x 256 and drop in the 2 x 512 making an upgrade of ½ a Gig (in each slot) giving a total of 1 Gig.
The probability is that there are four slots, two of which might be empty, its the combination making up the total that determines how much extra you can fit....
I hope thats clear??

BluRay