Q: will windows 8 work on a late 2009 27 inch iMac

I have a late 2009 27 inch iMac,  3.06 GHz Intel Core 2 Duo, 4GB RAM

I recently upgraded to Yosemite, 10.10.2

I have previously run Windows XP in Bootcamp

I need to upgrade my Windows because my banking software upgrade no longer works with XP

The questions:

Am I limited to which Windows I can get, 7.8. 8.1?  Will any of these work?

A few weeks ago I used a link I came across while browsing the Apple support page on this topic, it analyzed my machine and recommended that I get Windows 7, 32 bit

But I see that the new Bootcamp 5.1 will only run 64 Bit

So which Windows should I buy,?   7,8,8.1?   and 32 or 64 Bit?

And what Windows drivers might I have on my machine with Yosemite or be able to upgrade to?

Windows is not cheap, so I do not want to buy something that will not work