I am upgrading a friends HP with Windows XP loaded on it. She only has 512MB removable RAM. I ordered 2X1GB of Kingston DDR 400 PC-3200. Im thinking it has 4 memory slots. Would it be a noticeable difference adding these 2 next to the 512, or should I replace these? Any suggestions...2.5GB or 2GB?

It has an AMD Athlon 3400+ Single Core. AND this board supports up to 4GBs.

upgrade the RAM.
the difference between 2.5 and 2 is large enough to keep the old RAM.

with more ram, the computer will be much faster-
the only other thing i would suggest, is if she uses it for gaming, to upgrade the Graphics card....

She just needs a major RAM upgrade. It takes forever for My Computer, Anti-Virus, or any type of program to load.

Also, I do have a sorta old EVGA 7950GT. If I let her use this would it help her performance any?

But no she doesnt play games.

what anti-virus do you have? Some take forever to load

She just needs a major RAM upgrade. It takes forever for My Computer, Anti-Virus, or any type of program to load.

Also, I do have a sorta old EVGA 7950GT. If I let her use this would it help her performance any?

But no she doesnt play games.

if her computer can support it, yes- it takes Graphics Processing AWAY from the CPU. (although 7950GT would be overkill..)