Possible Memory Riser Card Failure

After my MacPro (MacPro early 2008 3,1 2 x28.8GHz Quad-Core Xeon) failed to start and exhibited the flashing LED light on the power button - and running all the usual resets (PRAM etc) - I started the task of taking out the RAM one by one to isolate which FB-DIMM if any was causing me problems.

During the process I noticed that if any sticks occupied the 1st 2 slots in Riser B, the computer failed to turn on and the light on the power button would flash.

Well with those 2 slots empty I was able to restart my machine however when I looked at the system report, the computer wasn't even reading the memory in Riser B inthe 3rd and 4th slots.

Another restart using the Apple Hardware Test on my install DVD resulted in all tests being passed but still not reading any of the RAM in Riser B.

One last thing I tried is I switched the position of the cards (I put the riser that was in the top riser on the bottom and the bottom one on top.) Yes the machine was able to start, but again as long as slots 1 and 2 on the suspect card were empty.

And when I checked the system report, this time it was not able to detect any of the RAM in Riser A (the riser card, I had previously had in the lower position) but was able to read all ram in the Riser B. So it appears to be isolated to that Riser Card.

Should I just go ahead and replace the Riser Card or is there anything else I may have missed? And is this a user replaceable part?

I've owned this machine for several years now and applecare/warranty has lapsed but I am willing to spend money to repair.

Any and all suggestions appreciated, Thanks