Here is a link to the Mac Pro early 2008 user manual:
http://www.apple.com/support/manuals/macpro/
In response to your questions, as long as the module in riser A slot 1 is an exact match of slot 2, and the module in Riser B slot 1 is an exact match to slot 2, and so on, then you are fine. The manual says you should add modules in the following order:
Riser A: 1&2
Riser B: 1&2
Riser A: 3&4
Riser B: 3&4
Provided you fill the risers in this order, and that you add them in matched pairs, then you can add any combination of module sizes you like.
So, to answer your question, you can add 4 x 1GB for a total of 6GB (remember to move the Apple module from Riser B slot 1 to Riser A slot 2 before adding that extra ram to Riser B slots 1&2, and Riser A slots 3&4).
Lots of people have recommended OWC for Mac Pro memory, it uses Apple approved heat sinks.
Barefeats have shown that filling all 8 slots will give the maximum data throughput, with any combination of module sizes. However, they also suggest that there
might be a performance benefit from having all modules of the same size (eg. 8 x 1GB or 8 x 2GB) although I don't think anyone has proven this yet.
Message was edited by: bilbo_baggins