According to Crucial, your system can take up to 4 modules of 1GB each (giving a max of 4GB in total).
The figures 123runner gave you are different (512MB/2GB). He has got these figures by looking up the actual motherboard in Crucials memory advisor. Normally I would expect this method to produce the same results but in this case, it clearly doesn't. The only explanation I can think of for this, is that Everest has detected the motherboard incorrectly maybe. Of course, I could be wrong....
Either way, both results say you have 4 slots (so 2 empty ones). The discrepancy is only with the maximum capacity of module your system will support.