dmidecode showing more ram slots than available?

Solution 1:

The chipset supports 8 memory slots. There's likely plumbing for 8 actual DIMM slots, but only 6 of them are connected. The motherboard manufacturer could have put 8 slots onto the board, but chose not to.

What's saying there's an error in "Bank 6"? Some kind of server management console? One of the slots is probably labelled as DIMM06 on the motherboard - I'd check that.

As for the size discrepancy, it's likely that you actually do have 4GB DIMMs installed, but a bug in your BIOS or DMI table is reporting them as only 2GB DIMMs.

Bottom line: don't trust dmidecode, it's not giving you enough information here. Visual inspection is required.

(or alternatively, check Tyan's website)