RAM upgrading, equal size or doesn't matter?

A friend has an older Sony desktop - PCV-RS100(UC)
2.00 gigahertz Intel Pentium 4
512 kilobyte secondary memory cache
Board: ASUSTeK Computer INC. KIRIN REV 1.xx


She currently has 248RAM running.
I'm assuming her board only has 2 slots.
She wants to upgrade and I told her to use matching sticks 2x256.
But she wants to grab a 512 to add to her already 256.
Would it make a difference as to stability for matching pairs or not?

Thanks.

 

Having a stick of 512 and a stick of 256 on the same board shouldn't cause any problems. Just be sure to get the correct kind of RAM, and be sure that her motherboard can support 512Mb per slot. (My guess is it will)