Memory capacity line: Supports up to 2 GB of unbuffered, non-ECC DDR2 667*/533/400MHz memory (Single / Dual Channel) as described in the link below.
http://www.supermicro.com/products/system/1U/5015/SYS-5015A-H.cfm?typ=L
That refers to one slot, which means the system can recognize 2G x 2 = 4G, or 2 slots all together, which means the system can recognize only 1G x 2 for a total 2G
The X7SLA-L/X7SLA-H supports up to 2GB Un-buffered Non-ECC DDR2 533/400 in 2 DIMM slots. That means the system can recognize only 1G x 2 for a total 2G