Some additional info:
1)Buy 4-core procesor (Core Quad 2.4 ghz- 400$). VVIS/VRAD effective use multicore systems as i know.
2)Overclock 4 core processor to 3-3,2 ghz
3)VRAD can be run with additional params:
-fast - realy fast VRAD, but lighning realy blocky and crap. Good for dev-mode.
-final - realy slow final light rendering for release. Maximum VRAD quality
, but hours and hours for compile.
-noextra - makes lighning bit poor, but speeds up VRAD. dev-mode.
4)Use VisGroups in hammer. You can group all what you need (if it non cause LEAK of course) and hide/unhide with one click. Objects, that hidden - dont render by VRAD and dont appers in game, it realy improve perfomance for VVIS/VRAD.
5)Of course dont compile with HDR - its makes 2x longer VRAD calculations.
6)Make good VVIS optimisation (by Hint-brushes) - its also speed up VRAD ambient lightning.
7)You can use cordon tool to cut little part of map and render it.