interesting. after doing a bit of searching regarding this issue, it seems that in other cases, the problem had been remedied by changing from two 512 sticks to one 1024mb stick. however, in your case, you're already using a single 1GB stick...
some of the other threads i have found in various forums suggests that matching two 512 sticks (revision #) solves the problem.
post back to ensure you have the single 1gb stick, just to be sure.