New MacPro - Bad RAM or Bad Hardware?

I'm having trouble with my RAM upgrade. I just bought a new 2.8 dual quad core MacPro with 2GB of RAM (2x1GB). I bought an additional 4GB RAM (2x2GB) from TransIntl. However, the most my Mac will see is 4GB, no matter how I configure the RAM. This is what I've tried:

Original Configuration:
Riser A1: Apple 1GB
Riser A2:
Riser B1: Apple 1GB
Riser B2:

Total memory visible by System Profiler: 2GB (1 in Riser A, 1 in Riser B)

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Apple Suggested Configuration:
Riser A1: Apple 1GB
Riser A2: Apple 1GB
Riser B1: TransIntl 2GB
Riser B2: TransIntl 2GB

Total memory visible by System Profiler: 2GB in Riser A, none in Riser B

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Alternate Test:
Riser A1: TransIntl 2GB
Riser A2: TransIntl 2GB
Riser B1: Apple 1GB
Riser B2: Apple 1GB

Total memory visible by System Profiler: 4GB in Riser A, none in Riser B

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I've reseated both risers and all the RAM but to no avail. After I tried Apple's suggested configuration, I ran the Hardware Test disk. It saw all the RAM, however it would only identify the Apple RAM with part numbers. Plus, it only calculated a total of 2GB. When I restarted, only the Apple RAM was visible in the System Profiler, all the other slots reported as "empty." I've returned the system to the original configuration (1 in Riser A, 1 in Riser B) and it correctly reports 2 GB total.

I thought at first the TransIntl RAM was faulty, but it does show up correctly in the Alternate test. Is it possible to load all the RAM on one Riser for testing purposes? Or maybe place them in different slots on Riser B?

No, I haven't tried that configuration.

Although it doesn't correspond with Apple's directions on RAM layout, I'm starting to think about trying different layouts, I'm just concerned about possible damage to the logic board.

This is a surprising turn of events...I went home last night resigned to dealing with either bad RAM of a hardware issue. I moved the Apple RAM to riser A and installed the TransIntl RAM into riser B. Now it's recognized, and the System Profiler even comes up with part numbers. The cynic in me says that I must not have seated the risers/RAM correctly, but I'm sure everything was locked into place originally, I did it at least 3 times!

I reset the PRAM for good measure, and started playing around with loading software. I'm not sure why the RAM is now recognized, but this is definitely a sure sign that AppleCare will be a required purchase! My next task is to run the Hardware Test CD just to double-check the RAM.