Ram compatability

My neighbor was throwing his old computer away so I snagged it.It is an eMachine T2482, AMD Athlon XP 2400+. It took so long to boot the first time I tried that I thought it was dead. I tried again later, was called away from my bench and when I came back it was up, but sooo slow. I checked for stuff in the start up cleaned that up and it got a little better. I ended up tossing all his files, and most of his programs, and got it to where it is running decently. I have decided to give it to my sister, she has an antique that barely runs. I opened it up and gave it a thorough cleaning and found it has one stick of ram >>Crucial 256Mb DDR 400 CL3<<< and one empty slot. I checked through my parts and found a ^^Samsung 256Mb DDR PC 3200 CL3 ^^ stick. Can I use that with the old one? If not, will replacing it with the 3200 increase performance? I also have two matching 128Mb DDR PC 2700 CL25sticks, would that be any improvement?
I really don't want to buy new ram for it, I will have to invest on a new WinXP OS, to be legal, so I'm trying to keep my cost down. I have a lot more labor than money.


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