"Sony Creative Software plans to release a 64-bit version of Vegas software by the end of 2007."

So is this version already avaliable? or soon to come?

Not available. Soon to come according to that press release.

Before answering the upgrade issue, the real question is what you need the speed for. Are in this for profit where your time is the issue? Since encoding is so time consuming (at least 3x running time on a dual-core), maybe the best thing is to have two independent machines so you can continue working on another project while one is churning away! For me this is (an expensive) hobby and after 6 hours of editing, I am more than happy to let the machine churn into the evening on some longish task.