Old Ram From Dell Inspiron 530 To Dell Optiplex GX280

I recently pulled two 512 MB memory sticks from my Dell Inspiron 530. They are Samsung PC2-5300U DDR2-667 512MB M378T6553EZS-CE600. Anyway, my mother in law has an old Dell OptiPlex GX280. According to Crucial Computer memory upgrades for Dell OptiPlex GX280 (Desktop and Mini-tower Models) Desktop/PC from Crucial.com the Memory:
DDR2 PC2-5300, DDR2 PC2-6400

Memory Type: DDR2 PC2-5300, DDR2 PC2-6400, DDR2 (non-ECC)
Maximum Memory: 4GB
Slots: 4
Each memory slot can hold DDR2 PC2-5300, DDR2 PC2-6400 with a maximum of 1GB per slot.*

But, I had her do a scan and she has one 256MB DDR PC2-3200 and one 512MB DDR PC2-3200.

My questions are:

-Are the old Samsung sticks compatible for her Optiplex? ( I think they are.)

-Since Crucial states "Although the memory can be installed one module at a time, the best performance comes from using matched pairs of modules." Should I simply remove her old sticks and replace with the two matching 512 MB sticks?

-Or, could I leave what she had and simply add the two Samsungs?

-Will the "new" Samsung" memory sticks speed up her computer?

It won't hurt to try. Just be sure you install them the right direction; the center notch is slightly off center. If the system doesn't post, then you can remove them. I use cpuz, a free download, to check speed and timings. pc3200 may only read as 400 with cpuz (half the actual speed). Your system will probably run at this slower speed even when adding faster modules.