The difference in performance only comes after you have 4+ gigs of ram (it supports up to... 128GB I believe) and if you want to see the difference between Home Premium and Ultimate just go to MSs website.
I really suggest to investigate if all of your hardware and apps support 64 bit. If they do, you should be able to so with little change in performance but you'll be ready for when 64 bit becomes prime time in about a year.
Things to do first:
1) Check to see if all of your hardware & peripherals have 64 bit drivers
If all of your hardware and peripherals check out and they all have 64 bit drivers than go for the x64 bit edition. It is in my opinion the future... Regarding Vista Home Premium and Ultimate, Home Premium doesn't have BitLocker and some other features. Look here.
Just so you know, when you buy Home Premium, it comes as 32bit. If you want the 64bit, you have to go to ms website and order the 64bit for an additional $10 and input your keycode from the home premium you just purchased. The info is in the box when you but it. I ordered it, but will probably hang onto it for a while before I load it given the driver situation. Ultimate comes with 64bit in the box.
The link you posted is unavailable I think due to the fact that they are out of stock until the 17th. But to answer your question, yes. I purchased Vista Home Premium from a local retailer and nowhere on MS website did it say about the 32bit vs 64bit issue. When I opened the box, there was a card inside with instructions on how to get the 64bit version. Doing some further investigation, I found out that Ultimate comes with the 64bit, but it is also $160 more US Dollars. I would imagine it would be that way everywhere. I would call before you order and ask the specifics. Hope this helps.
You have to specify which version you want with the OEM. It comes in either 32 or 64 bit editions. You don't have to pay extra for 64 bit, but the version you choose is what you'll be stuck with. If you change your mind you'll have to buy a whole new OEM license. Check out Anandtechs vista review. He explains it all in depth.
With the retail version you have to order a CD with 64 on it. Only Ultimate comes with both
True, but word is that MS is going to get really stingy with OEM licenses this time around. I just bought the full retail version because I forsee myself changing hardware and doing various reinstalls down the road. If MS enforces the OEM licensing like they did for XP you'll be set, but if they clamp down you may have to buy multiple licenses.