I'm referring to the T5400. I believe the 690 is a different chassis. You stated you already have one running (I assumed that to be the T5400), so you should be able to assess the situation yourself. The power consumption depends on how they are equipped. I would check the stats of the box (processor) and then do a Google search to see how much juice it will take. Two older processors might take 100 watts (I'm not sure), just for those, and when you add the other items (drives, cards, etc) your box could be taking 200watts or more. Also check how much drive space you have. I could be wrong about the 2 drive bay. I suppose it depends if you have a DVD-ROM drive in it and other items.
For me power would not be a major consideration as I live in a part of the country where it is very cheap. If on the other hand you live in Hawaii, where power is expensive, it might be a primary concern.