Virtualization has not come far enough to support high end 3d games. HL2 and CS will not run well, if at all. Your 3 year old PC would run it faster than one through virtualization.
DirectX 8 is the highest I know (VMWare), and even that is experimental. Someone chime in too if they've done better than DX8.
A true windows install through bootcamp would be much better for gaming purposes, and is the only way to run games at their full potential on an Intel Mac.
Just install windows with bootcamp.