upgrading rdram

I have a Gateway Performance 1400 with 128mb rd ram pc 800.
I have purchased kingston rd ram 256mb pc 800 from bigegg.com.
Bigegg offers no tech support, and everyone at work in the IT dept says it should work fine. But do I have to take out the old, insert the new,and move the old to a different slot. I have 4 slots for mem.