PSU orientation and position

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Are some PSU's designed to be mounted at the top and some at the bottom of the case.
My Thermaltake (which I want to replace) has the fan pointing down and is ideal for my top of the case design. I see that most show the fan facing up. That suggests base mounting. As the screws on the back of the PSU are not in line, I cannot turn the PSU upside down.
Am I right and where do I find out which one is top mounting?

 

[foreright]

Thanks guys..........

Can you also tell me what size is recommended. I currently have a 450W, but I don't know if that was calculated for or is efficient. As I have already said, it's awfully noisy.
I want to get one that is efficient, not too big, not too small, very quiet. Expense not important.
Thermaltake web calculator states that the demand for my PC is 280 -300W just a simple PC for office and browsing and a bit of media streaming.
What size would you get?