you should be able to run at least 3 characters at once on that hardware in windowed mode. using different monitors... dunno.

i can run 3 characters on my laptop (1.5GHz amd, 1GB, 64M ATI radeon mobility) and still get 20-25 FPS. the video on my site was on my desktop... 5 characters on one machine (2GHz AMD, 2GB memory, 256M nvidea). all coordinated with a single keyclone