My home comp has 4GB of DDR2 RAM and runs vista64bit...I rarely see it use more than 2GB of RAM when gaming + virus scans + firefox tabs in the background.
My main rig with 6GB of DDR3 RAM use no more than 3GB of RAM when I multitask and game like crazy.
I'd say get a good quad core (from your budget, I'd say get a AMD PhenomII quad, since they have the best CPUs for the price under $200) and 4GB of DDR2 or DDR3 double channel, or 6GB of DDR3 triple channel RAM will do just fine.
It depends on how intensive your audio mixes are going to be - 8GB RAM is more like video editing levels of RAM so you may be better off with only 4GB. Most effects are CPU-based, but if you're layering a lot of post-production on then it'll eat RAM.
That being said, RAM is fairly cheap again and with 8GB you have more than enough to go round to keep your system running regardless of what you throw at it.