How many until slow/Stop

I have a intel pentium dual core 2.8ghz and a 1gb ddr2 memory. I've tried running 3 different browsers at the same time, Firefox, Internet Explorer and Opera, and I noticed that my pc began to slow down. I don't know if it was because of those applications or because I have some background application running. Like, I always have Trendmicro's Internet Security 2007 running in real time, the HP digital Imaging Monitor and I think I was also downloading some files from Azureus. I never want to run too much application until my pc hangs because it might damage my pc.

Totally depends on the application, lightweight applications I can run like 50 or so till it begins slowing down. But in example when using Maya for rendering a set of images on how quality that application on itself is enough to slow my computer down to 0. AMD Turion 1800+, 512 DDR