I have two memory cards a Sandisk 256 OEM and a Trascend 512mb 80x and there is a lot of difference between them in the writting and reading speed

!! reading and writting are done faster in the 80x card.
If you want light speed operation, a SanDisk Extreme III SD cards are the clear winners there.
Also remember that the D50 can not use SD HC (new SD cards standard that go up to 8 GB)