sli is Scan Line Interleave .. the only card i know that does this was like the voodoo2 or something.. its kind of like load balancing with Video cards .. you take 2 voodoo2's, and one will render the odd lines, and the other will render the even lines .. i guess you could search for a more technical answer, but thats basically what it is ..
The MX .. well i could go over this, but look in AT's Video section .. basically, there are differences in rendering pipelines, core clock/memory speed, and of course between the GF2 and GF3 its a very different GPU .. hope this helps some .. good luck ..
-neural