laptop memory

My friends latop that I am working on is an Averatec 6100 series. Right now it has 2*256@DDR333, it has a P4 2.8, I suppose it will have 2*512, but wanted to know if I could bump it up to DDR400?

That aside, what memory would you all recomend for it, Im sure that the 333 and 400 speeds will be similarly priced. I think that it has a Sis chipset, Im not sure right now, because the windows install is all screwed up right now, and I am waiting for an adapter to pull some data off before I reinstall. Will post later, but,

What would you recomend, and how much of a performance increase can I expect going from 333 to 400? I think that the fsb is 800, but it may be 533, if it is 533 wouldnt I still get a little bit of an improvement, and if it is 800, then I should get a nice improvement, Trying to speed up his laptop a little, just make it a little snappier and keep pf usage to a minimum.