If your upgrading to a P4 2.0 Ghz system or better, I have one thing to say: DON'T SKIMP ON THE RAM!
It would be like buying a Ferarri body and putting a Volkswagon engine in it. Well, okay maybe not that drastic, but close...
If your getting a brand new system, don't go less than 512 MB DDR SDRAM, and if you can afford it I would go with the 1.2 ghz ram option instead. This type of ram can process twice the info in the same amount of time as the old RAM. If your buying a new machine I am sure it will be in it, I don't think they use the old RAM much anymore in new machines. If you can't swing the gig of ram, then go with 512 and make sure that you have another slot on your board so you can add it later.
The relationship of processor and RAM is very important. You can have a kick ass processor, but if it doesn't have the RAM to support it you will find your system will hit it's limits earlier. You will find you can't use as many plug-in's as you might want to because they are overloading the system. You won't be able to run as many VSTi's simultaniously. You will find that your simultanious recording and playback of audio tracks may become unstable and glitchy. The problem is once you discover there is a problem, it is usually at the most unopportune time and adding hardware is a pain in the ass when your in the middle of a project.
Anyhow, I run a 1.9 ghz P4 with 512 DDR SDRAM and it runs smooth as can be, but I am still planning on upping it to 1.2 ghz in the near future. Remember, as tech is getting better, it makes programs and plug-in's and vsti's more "hoggy" of your systems resources. I decided to build a system that would last me for years to come and not one where 12 months from now I am calling it a piece of junk because it can't do what I want.
Good luck and have fun making trouble free music! Go 512 and above.
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(all this giving good advice and now twice in a week...two duds) 
