I have a w500 ThinkPad (product id = 4058CTO) and according to the ThinkVantage system information I have 4gb ram with "0 remaining DIMM slots"). Does this mean I can only upgrade to 8gb by buying two 4gb ram sticks (i.e. there are no 6gb sticks)?
true. The sticks are all "powers of 2", ie 256 mg, 512mb, 1gb, 2gb, 4gb, etc.