Ram upgrade. I was wondering how many MHz my desktop is because I wanted to get the ram up

Question

  I was wondering how many MHz my desktop is because I wanted to get the ram up to 2 gigs.  I had it up to 1.50 gigs with the stock ram and a K-byte DDR2 1024 pc 4200.  I took out the stock ram and replaced it with one ultra DDR2 1024 pc2-4200 533MHz and the K-byte DDR2 1024 pc 4200.  I can't seem to get 2 gigs because the system properties is saying there is only 1 gig.  The max gigs this motherboard can handel  is 2 gigs. The ram isn't defective because it will read 1 gig if I pull one or the other out of the slot.  Maybe its because there not the same brand idk.