Ok, since the Mac Pro comes with an entry level 2 x 1gb, wouldn't it be best to buy 6 x 1gb cards so that you would have 4 x 1 gb on the bottom and 4 x 1 gb on the top? From Apple:
Mac Pro (Early 2008)
The FB DIMMs used in Mac Pro (Early 2008) computers are 800 MHz DDR2 ECC fully-buffered DIMMs. The Mac Pro (early 2008) has eight FB DIMM slots on two memory riser cards that support up to 32 GB of main memory. The DIMMs are installed as pairs of 1 GB, 2 GB and 4 GB sizes.
Solution
This table shows Mac Pro RAM configurations that will achieve ideal performance when running Final Cut Studio applications:
Riser A contains: Riser B contains:
2 x 512MB 2 x 512MB
2 x 1GB 2 x 1GB
2 x 2GB 2 x 2GB
4 x 512MB 4 x 512MB
4 x 1GB 4 x 1GB
4 x 2GB 4 x 2GB
Note: DIMMs must be installed in pairs of equal size from the same vendor. For instance, you must not have one or three DIMMs on either riser at any time. Additionally, two DIMMs from different vendors should not be combined and used as a pair.
Mac Pro (Early 2008)
The FB DIMMs used in Mac Pro (Early 2008) computers are 800 MHz DDR2 ECC fully-buffered DIMMs. The Mac Pro (early 2008) has eight FB DIMM slots on two memory riser cards that support up to 32 GB of main memory. The DIMMs are installed as pairs of 1 GB, 2 GB and 4 GB sizes.
Solution
This table shows Mac Pro RAM configurations that will achieve ideal performance when running Final Cut Studio applications:
Riser A contains: Riser B contains:
2 x 512MB 2 x 512MB
2 x 1GB 2 x 1GB
2 x 2GB 2 x 2GB
4 x 512MB 4 x 512MB
4 x 1GB 4 x 1GB
4 x 2GB 4 x 2GB
Note: DIMMs must be installed in pairs of equal size from the same vendor. For instance, you must not have one or three DIMMs on either riser at any time. Additionally, two DIMMs from different vendors should not be combined and used as a pair.