Vegas 4 is not well optimized for multi-processing. But Vegas does allow you to you run multiple instances. So 2 CPUs will help running multiple instances of Vegas. Each instance will be more responsive than on a hypertheading CPU.
Some processes in Vegas do work very well with multi-processing. I believe rendering to mpeg2 and Windows Media 9 will really use multiple-processors. So if you muli-task a lot or go to MPEG2 a lot, then you may want to consider 2 CPUs. But with Vegas 4 you don't need 2 CPUs.
Someone on this board ran some tests with hyperthreading and found that hyperthreading allows for a 12% boost in Vegas. Vegas does the audio as one thread and the video as another. And hyper threading is virtually free so to me it's worth it.
Hyperthreading is great for multitasking, I think. I have a 2.66Ghz Pentium and 1.25 Gig of RAM. Just now I have 2 instances of Vegas rendering WM9 to one hard drive and another instance of Vegas rendering to AVI on another hard drive with Word and a few IE windows open.
So to me, a fast Pentium 4, with a lot of RAM and front side bus of 800mHz is a nice system.
You should look around and do some more reading on HT. The basic thing to remember is that Vegas will keep a CPU running at 100%. Since HT alone is still just one CPU how much could it do for you? 100% is 100%. What it can do is make your system generally more responsive.
It won't be long before every intel CPU you would consider for Vegas will have HT. The question will then be whether to go into BIOS and turn it off, if you can.
Some articles I've read recently say that HT really becomes useful when you're running very fast ram on a very fast bus. I think dual channel memory setups were helpful too.
Even if HT doesn't help directly, any of the fastest Intel CPUs will have it. Even the Xeons in a dual processor system. So you'll get it no matter what. And it'll help with overall snappiness. Maybe you'll find that it helps the editing performance more than the rendering performance.
I think Rob's hit the nail on the head. From what I can see it's getting harder to buy a P4 without it so it's rather academic.
From what I can see it doesn't make Vegas run any faster, well not this version anyway. But if you run 2 instances of Vegas they both run at about the same speed as one instance. If you've got a lot of rendering to do and can plan your workflow around how HT will help you then I'd say its definately worth it.
In my machine with P4 2.8 with HT I get a 12.4% improvement in rendering MPEG2 DVD and the processor shows 50 to 80 %. The processor temp jumps from 105F to 131Fduring the render. I am very happy with the improvement and it really adds up if your doing a 1.5hr clip.
By the way, if you are thinking of getting an Intel Chip, I just saw on Google News they are cutting prices on Monday a few weeks earlier than they said they would. I don't know how long that will take to show up at the retail level, probably a week or so.