Mac pro memory problem

Hi,
3 months ago I bought a Mac Pro from online apple store and I just noticed that in the 'about my mac' window 3 Gb memory was reported whereas I have 4 Gb present. There are 4 x 1Gb DIMM's present. 2 on Riser A and 2 on Riser b, same positions, DIMMS 1 and 2. I pulled them out to see if they were all corectly attached and they were.

In the system profile at the tab diagnostic info this is stated:
Laatst uitgevoerd: 24-07-08 17:29
Resultaat: Fout (error)
Type fout: Geheugen (memory)
Geheugensleuf: DIMM 1/DIMM Riser B
It's in dutch but it says there is an error in DIMM1/DIMM Riser B. The memory tab window shows this:
DIMM Riser B/DIMM 1:

Grootte: 512 MB --> same for DIMM Riser B/DIMM 2
Type: DDR2 FB-DIMM
Snelheid: 800 MHz
Status: OK

So the 512 Mb is present in DIMM's 1 and 2 of Riser B whereas they really are 1 Gb each. So I did a hardware test (2x, including the thorough one) but no errors were found. When I then look at the test at start-up it states that there is a memory error in DIMM1 Riser B.
Could it be broken or is it just a bad report?

 

Nice solution . Tried it and the problem shifted to the A Riser so it must the memory. I'll visit an AASP. Thx for the advice!