You don't mention which version of Windows you're using so I'll assume XP. XP uses a lot more of your RAM then Win2k did, so you might want to make sure that you've eliminated as many non-essential running-in-the-background programs (used to be called TSRs, but I'm not sure of the current nomenclature) as you can. The way to do this is ctrl-alt-del, then click on Windows Task Manager. Then click on the processes tab. For the most part, you don't want to just stop any old process willy-nilly--you need to figure out what they do. I don't have the resources handy, but I'm sure a Google search would turn those up. But as an example, I'm looking at my current [non-video editing] setup, and I see an iTunes process which I could harmlessly stop (and which will restart when I reboot) which takes up 1MB of RAM.
When you're done stopping the items you don't need, and I emphasize how much care you should take in this procedure again, click on the performance tab. Looking in the bottom right portion of the window, you will see your physical memory. You should try achieving the maximum amount of "available" RAM through the process outlined above. Obviously, the system requires a certain amount of minimum RAM to function, but that is specific to the machine, drivers, etc. that you have, so no specific number can be given. I do know that Win2k's minimum is better than XP's minimum and it is a significant difference.
There are some other ways of improving your performance for Vegas, and these are actually true for any OS and any real-time rendering NLEs (like Final Cut, Premiere, etc.). The uber-system has max RAM (which it sounds like you probably have), but also has at least 4 dedicated hard drives (or RAIDs): one for the OS and software, one only for the Windows (or other OS) paging file, one for video, and one for audio. You might also want to add one for final output as well. This way you don't suffer a performance hit every time the drive is accessed for each of those elements in your timeline.
My paging file drive is only a 20GB drive, with a fixed 10GB paging file. Using Vegas, I have been able to watch the fully-rendered (though draft quality) video in real-time of up to 8 layers, and I only have a 1.3ghz, p4 (but w/4 dedicated drives and 1GB RAM).
The quality of the video card is important also, though not at the level of a gamer's machine. I actually wrote an article for one of the PC magazines which compared various video cards for NLE purposes and found that the best card (meaning bang for buck, with NECESSARY features) was the Matrox G550 64mb dual head, though that was a few years back. I am still using that card and it still serves me well. I'm phasing in a new 3ghz PC and am currently trying a GeForce 256mb with dual monitor tech. I haven't done a lot of editing with this machine, but it seems pretty snappy.
Hope that long answer that covered other speed items was of help.
DT