I'm running debian etch on a pentium 3 930 MHz with 378 meg of ram, a nvidia graphics card (with nvidia binary drivers) and a PCI sound card (with hardware midi) I can do everything: Multimedia, streaming or otherwise, documents, full KDE GUI, web browsing, E-mail, and games but I doub't I could play Quake4 or anything that demanding, but then what are high powered windows machines for
Find another 200 meg of ram and you're set
You can get debian etch here
, I recomend typing installgui at the boot prompt to try the new installer. I also recomend not choseing a Desktop environment when given the choice because thats gnome. just go without, log on to the command line (as root) and type
apt-get install xorg kde kdm synaptic
If you actually like Gnome just select Desktop environment.
One final warning, if you chose debian you'll have to manually install binary graphics card drivers, non-free codecs, flash, java, etc. just ask around if you have any trouble.