Vegas is multithreaded. Even when rendering. But as a general rule, a second processor only gives you about 50% improvement at the most. And also it will give you more responsiveness when using multiple programs. However, the new HT CPUs provide much better responsiveness in multitasking even under a load, so it makes that reason a non issue.
The big thing is that the fastest P3 out there is only 1.33 GHz. And the fastes P4 is 3.2 GHz. That's quite a bit faster.
Consider one that allows easy overclocking from the BIOS. Mine, a ASUS P4P800 Deluxe does by making one simple change. The benefits really pay in longer projects with improved rendering times.
okay.. so I built my own system but a computer jean-yus I am not. I'm running a P4 1.9 on a ASUS motherboard. Would overclocking provide me enough extra performance to make it worth the risk to my system.. or is there even a risk to OCing anymore?
The only risk to overclocking is overheating. At least in 99% of all cases. If you can keep the motherboard, processor, memory, and graphic cards cool enough you could crank the speed up to 3GHz. But unless your motherboard has on board temperature sensors I wouldn't recommend that you try.
There are different kinds cooling systems you can buy besides the normal Fan/heatsink method. There are plenty of web sites that cover the ins and outs of overclocking. If you want to try it I would do a google search and read a few of the more recent of these types of guides.