Try it... 512MB RAM is probably plenty, if you have a dual channel capable board use 2 256MB sticks for best results. If you need more, the breakpoint seems to be 768MB, and a couple of 128s are cheaper than another pair of 256s. If your MB uses single channel, start with a 512 and add a 256 if necessary.
For a video card, an AGP card (either NVidia or ATI) will outperform an embedded, especially if you will be using it with a frame grabber or genlock card to capture streaming video from NTSC or PAL tape and VCD players.
For an OS, start with either XP or 2K, whichever you prefer. If it dumps out, it's still good enough to download a Linux distro that will work...
Most front panel USB ports will work OK as whatever the MB and OS supports on the MB or card connector they are wired to. If they are connected to an MB USB2 connector and run with WinXP or latest Linux, they should be USB2...