Nothing is wrong with mixxing differnt type of RAM at all.
BUT mixxing PC2100 with PC2700 may limit your speed to 133FSB. The PC2700 may be rated for 166FSB, but can be held back from the stick of PC2100 in the system.
And as for no name and name brand RAM, once again, no problems should come of it. As if the RAM has been made according to JEDEC, its all the same.
Overclocking is were the difference MAY show. Gernic RAM may not be able to run as fast as Micron....
Or may not like to be installed with other large amounts of RAM when runnign out of spec as the manufacturing is not as good as the name brand.
Also the more sticks of RAM in a system, the lower your RAM can be clocked. And this often shows up more using different brands, just another thing to think about.
DDR400 (PC3200) being mixed with other RAM may however cause problems. This is because PC3200 is not a standard and has to "SPEC" so the quality and compatibility are all unknown.
But as long as you are running mixed stick at the speed that the lowest stick is rated for (stable at), you shouldnt see any problems.
Don't forget that one brand of ram will respond differently to more voltage than another. Micron loves voltage, and can take quite a bit, while boosting performance quite a bit. Other ram types may just run hotter or burn out quicker.
DDR SDRAM is generally safe to run at 2.8V (from 2.5V)
SDR SDRAM is generally safe to run at 3.6V (from 3.3V)
just follow the 10% rule of thum when playing with voltage...
and that is, if you stay withen 10% of spec the odds of voltage killing anything is low.
At 15% things start to get risky.
At 20%+ the benifit of raising the voltage quickly disapears, and the liklyhood of dead parts is VERY high.
Its the same rule you use with CPU's