DDR memory runs at twice the speed of the memory bus clock (Double Data Rate).
Your new RAM is thus specified to work with a 133MHz clock. Normally you would expect it to work OK with 100MHz too.
BUT mixing RAM speeds may lead to instabilities too - there's no hard and fast rule on it.
You should be able to mix capacities wihout problems - though it's best practice to put the highest capacity in the lowest numbered slot.
So try changing your RAM over - 512 in A0 - 256 in A1.
Does the system run OK with just the 512 module in the A0 slot.
Somtimes, also, unused slots may be a little dirty - push the RAM in and out a couple of times.
Ideally use only the same brand and speed of RAM together.