Have you played the original Half-Life? Gamespot just added it to its list of Greatest Games of All Time.  I played HL and HL2 back to back.  Going from DirectX 6 to DirectX 9 is crazy.  Shows how much games have improved visually, in under a decade.   HL2 has been out for almost 3 yrs now, so its better optimized and a lot less buggy than S.T.A.L.K.E.R.  Can't wait for Team Fortress 2 and Portal.  Orange Box FTW.