Hmmm... I think thats not a good solution to use 2 different manufacturer...
In past I had the same problem with 1x 512 MB and 1x 256 MB... In the end, the motherboard turned on the wrong values (the settings from the 512MB) and my 256 MB said "Goodbye"
If u use 2 different, check if the values from the lower sticks are set!