I'm not sure I would run 15+ at one time (though I do manually activate/deactivate projects to give them turns on my Athlon 64, which is incidently also a 3500+). However, you can definitely run more then 2 projects and not run into problems. 5 or 6, or maybe even 8 wouldn't be a prob on that proc.
As to memory, it depends what programs you're also running, and the system setup. I just went from Windows XP Pro, to now running Windows Vista Ultimate, beta 2. I can say first hand that Vista uses a crap load of memory compared to XP. Rather then booting up using 250 MB or so (on a clean install); it's closer to 800-900 MB. When people get Vista, they're likely going to be wanting 2 GB of RAM. With dual core procs and running DC WUs on both CPUs simultaniously, I could see them wanting more. AKA, seasonal attribution on one core, and CPDN coupled ocean on the other
Projects can be written out to the paging file, and in this way it can help. The inactive WU can be paged out, and because it isn't being worked on, it doesn't have to be in system RAM. There is still load times, but eh... Now, looking at things on Vista, I can say that when things go a few hundred megs over physical RAM (not difficult even with a gig, when the system boots up using over 800 MBs to itself), that things can become a bit slow/unresponsive. Now that might be a Vista thing, could also be a beta thing. As long as it's below system RAM, it's actually a bit more responsive then winXP, probably in part because I'm using Vista-64 bit beta, rather then the 32-bit variant.
More readily, yes there's a lot of computing power on the A64, but it isn't inexhaustible. And yes now, with the projects I have (and certainly not everything), an x2 (dual core) would help :rofl For now, I just micro-manage things a bit to cycle through some projects I want to run at the time, and set others to no new work, until I get back to them. I do run groups of projects at a time however...