Errors and incompatability problems are common when you try to run 2 different brands of RAM. They aren't made to a specific standard just because they have the same numbers on the packaging. This might be especially problematic if you are trying to run the RAM in dual channel mode. I managed to get away with a 256mb Kingston PC3200 and a Corsair 256mb PC3200, but I never risked pushing them too hard. Furthermore Kingston and Corsair are two of the most respected memory manufacturers...certainly a little more reputable than 'Hynix'...did you get it from PC world or something?
Erm. Are you sure its on the motherboards supported DDR list? It doesn't usually HAVE to be, its just sometimes memory ISN'T on there for a reason..
When you go into the motherboard bios, are the RAM voltages what they should be? Are the latency timings the same as the RAM manufacturer suggests? That might sound overcomplicated, but basically set it to 'set to spd' wherever possible and it'll sort itself out.
If I were you I'd just buy a single 512mb PC3200 from Crucial, Corsair, or Kingston. Its faster, less problematic, and only about 30-50 quid... just sell or chuck the old ones.
