Downloading I'm not sure. I think you're stuck. You could try contacting them. If you had purchased it retail, then you would have gotten both versions.
My question is, why do you want to go to 64-bit? Do you have some program that requires it? What are your system specs? If you're mainly gaming with 32-bit programs, then going 64-bit won't gain you much if anything.
What are your reasons to want to go to 64-bit and why do you regret going 32-bit?
Then by all means, I would say go for it. You won't really see any speed improvements on your games, but at least you'll have better stability, memory management and have access to all 4 gigs of your memory.
My machine isn't a monster like yours, but I still run 64-bit and it's been pretty stable. No BSODs for me except when I decide to try the occasional beta driver.
That's what I meant. Contact MS and see if there is a way for you to also obtain the 64-bit version without an extra charge. I don't know how the D/L portion goes, though. I assume you purchased it through Microsoft's store? The product description there states you get both. Check your email from Microsoft after you purchased Windows 7. It might have the instructions there on how to obtain both the 32 and 64-bit versions.
He purchased his copy online from Microsoft's website and chose the 32-bit version. Therefore, he doesn't have an actual retail DVD set that comes with both versions. I don't know how the process works with Microsoft concerning downloaded versions as I purchased my copy of Windows 7 through Amazon when they had the 50%-off sale.
Again, your best bet is to get with Microsoft and see if you can obtain the 64-bit version as well.