Basic question about Vista 64

[mcwildcard]

I want to get Vista 64 Home Premium, but the place I want to buy from (I have credit notes for there) only does the 32 bit version (retail).
Maybe I've misinterpreted what I've read previously, but are they actually the same package, with instructions on how to go up to 64 bit included?

 

[mcwildcard]

The full retail version has a free upgrade in the package I think. Bought V 64 oem myself.

They don't do OEM unfortunately, otherwise I'd go for that.
I should probably think about it a little more and do my sums, retail is a lot more £££, regardless of the credit I have.