The 32-Bit RAM thAng ~ 3GB vs 3.5GB?

Vista Home/Basic (32-Bit), with Crucial reporting this:

Your Inspiron 531 System Specs

Scan Id: 7881AC9AXXXXXXXX
Memory:
DDR2 PC2-5300, DDR2 PC2-6400

Memory Type: DDR2 PC2-5300, DDR2 PC2-6400, DDR2 (non-ECC)
Maximum Memory: 4GB
Currently Installed Memory: 3GB
Total Memory Slots: 4
Available Memory Slots: 0

1GB
1GB
512MB
512MB

Each memory slot can hold DDR2 PC2-5300, DDR2 PC2-6400 with a maximum of 1GB per slot.*
*Not to exceed manufacturer supported memory.

Crucial RECOMMENDATION:

1GB
DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 128Meg x 64 • • Part #: CT700772

1GB
DDR2 PC2-6400 • CL=6 • Unbuffered • NON-ECC • DDR2-800 • 1.8V • 128Meg x 64 • • Part #: CT763014
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Now, I "think" Crucial is telling me that I have two slightly different types of RAM here, though both are "acceptable" (accepted by the machine).

My question is two-fold (if not three):

1. They are suggesting I replace both (each) 512MB stix with a 1GB stik...is that really going to be cost-effective, seeing how the most I am going to access is "maybe" another 512MB, taking me to 3.5GB from the total of 3GB I now have? In fact, I "may" be MAXED-OUT already, who knows??

2. I listed Crucial's rec's in order given/supplied...is this to say, that DDR2 PC2-5300 • CL=5 DDR2-667 is "better than" DDR2 PC2-6400 • CL=6 DDR2-800, please?

And I have no clue now, what my 3rd question was, or perhaps should be? I suppose it was/is my comment before the first question: I "think" Crucial is telling me that I have two slightly different types of RAM here, though both are "acceptable" (accepted by the machine) - is this correct, please?

Thanks in advance,
g . . .

 

Only one answer needed, leave it all alone as what is suggested by Crucial will not make any perceptible difference to you except for a lighter wallet.