Activity Monitor showing significantly less memory than About This Mac
I would remove the two smaller DIMMs and see if the Activity Monitor sees 4GB, as it looks as if the 2 2GB DIMMs are not being seen. If this is the case, then keep the 512MBs out and upgrade the RAM, which is very inexpensive these days.

If you're planning to keep this Mac Pro for a while, its worth loading up on RAM even if you don't think you need it, because future OS version may require it. I've always tried to match RAM sizes in all slots, and not mix sizes, and even further, matching the DIMM manufacturer. This may just be an unneeded caution, but you should find out the exact RAM requirements for this particular Mac Pro on the Apple support site.