Second memory slot no longer recognized

Background:

I have a Toshiba Tecra 8200, until recently working with 2 x 128 PC100 SODIMMs (256MB).

I ordered new memory to try to take it to 512MB: 2 x 256MB PC133 SODIMMs on the basis that casual advice suggested the 133's would down-clock.

I installed, but only the first slot would be recognized.  I swapped them, only installed one, installed one 256 + one of the original 128's, and most other combinations.   Any combination where the 256 card was in the first slot resulted in only the first slot being recognized.  (Tested by holding the esc key while powering up, and then pressing F1 to get the bios config screen).

I assumed this had to do with a sensitivity to PC133, so I returned them and orderd 2 x 256MB PC100 SODIMMs.  However, after installing this second batch of memory the laptop would not boot at all, displaying a "remove incompatible memory" error.  I suspect they were cheap memory, and the tecra is just too finicky.

So, I reinstalled the original 128s, and ordered "proper" (i.e. recommended on the Toshiba site) 256MB PC100 Kingston modules.  (Still waiting for delivery.)

However, I noticed the laptop really crawling, much slower than originally.  I checked the memory and only 128MB was shown as installed, even though I had restored the two original SODIMMs.  I swapped the cards to make sure there wasn't a problem with the memory (including installing each one individually and then together) and in every case a max of 128MB was recognized.

Questions:

Is my 2nd slot toast, and my assumptions that I had incompatible memory originally a red herring?  Any advice on how to test this assumption and/or fix things up is much appreciated.

A possible resolution (which Toshiba doesn't state is feasible or not): if one slot actually is toast, can I put a 512 SODIMM in the first slot?  Or, will the system just not recognize it?  The tecra 8200 can take a maximum of 512, but I originally assumed that it had to be spread across both slots.  If I can do the full 512 in one slot that would be great (and cheaper than buying two 256's) but I don't want to go through the hassle of ordering and returning if one of you know it won't work outright.

There is some urgency to this as I may need to replace the laptop, as it is unusable in its current state.  BTW, I have contacted Toshiba directly on this, and in Canada I am simply referred to a memory supplier partner as the 8200 is outmoded, and in the US I could tell that the tech. specialist was reading from the same manual that I had just downloaded from their site.

Thanks for any help.  

Paul