If you decide to buy a full version instead you can buy a brand new Ultimate retail version for probably as much as you would for an upgrade on ebay. That way you could install it on any other computer you had as well. I believe when you buy a full retail version of ultimate it has both 32 and 64 bit disks.
If I could just clarify matters here ....
... Although you do indeed receive Both 32 and 64 bit DVDs with the full retail box of Vista ultimate - You only receive one
licence which may be used to install either
32 or 64 bit Vista Ultimate on One