1.  Get steam and get HL2, HL2:1 CS:S and Garry's mod 10. 

 2.  Get BF2 (Because more people play it and its cheaper than BF2142

3.  If your rig is good, get Rainbow six: Vegas.  Great game and shows off that Unreal Engine on the PC.

4.  Get CoH (Company of heroes)  Great RTS w/ no choke points.

5. Get Warcraft 3 and expansion pack.  Its cheap now and you wont waste your life like WoW.