Getting upgrade - advice needed

[theMAD2]

I need to upgrade my memory and I just want to make sure I get this right. I've currently got 512mb (2 x 256mb) DDR266 DIMMs running on my Intel DG865GLC mboard and I want to see my options for upgrade. They're running in dual-channel because I've put a pair in, so can I put another pair in running at DDR400, or will they just clock down to the slower pair?

Any advice appreciated

 

[theMAD2]

So I guess it would make more sense to replace the current chips to get much faster speed. Do you think it will be noticeable?