Compatible. I use 32bit vista on L4D. Are you experiencing any problems?
L4D is not really heavy. Since it uses a modified Source Engine used by Half-Life 2. HL2 uses Source Engine 1, as far as I know L4D uses Source Engine 27 only with AI improvements, workarounds, flycodes, etc...
You don't need to have steam to join dedicated servers here in the country. Since you can use the console to play directly to them.