Hi,
If you're using 64-bit (aka 'x64') now and want to continue to use 64-bit, then naturally you would get the Windows 8, x64.
There is no price difference between the 32 and 64 bit.
Even though years ago the advent of using a 64-bit data path was supposed to make computing much faster, realistically it hasn't done a whole lot (some, yes, but not what they expected would happen with it).
One of the problems is that software developers have to write the program to utilize 64-bit and not a lot have done it, even now in 2013.
I'm looking around on my desktop, with around 30 programs installed, and right off the bat I only see Photoshop that's 64-bit. I'm sure there are a couple more but not many.
HOWEVER
Despite all that, you definitely want a 64-bit operating system.
How much RAM it will let you utilize is the HUGE issue.
32-bit can only address right around 3.5GB. I say around because some computers report 3.42, 3.61, etc.
With 32-bit, even if you have 16GB of RAM in your system, it can only use 3.5.
However, the operating system itself (regardless of 32 or 64-bit) has an impact on useable RAM.
For example, (all 64-bit versions):
Windows version: RAM (memory) limit
Windows 7 Home Basic 8GB
Windows 7 Home Premium 16GB
Windows 7 Pro (and higher) 192GB
So as you can see, 64-bit is very desireable, memory-wise.
I stated a couple of oversimplified things about 32 vs 64-bit at the top, but if you're technical-minded, here's all about it:
http://en.wikipedia.org/wiki/64-bit_computing
LASTLY
As to your "Additional Details" listed, yes, the upgrade has to be the same as the version you're using or else you'll have to pay extra for a 'higher' version, i.e. if you have Windows 7 Home Premium, you have to get the equivalent Windows 8 Upgrade.
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For further clarification of my answer, if needed, contact me directly.
At your service,
Hugh
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