Personally I would wait and see if they offer the STALKER series at some super cheap price - that is the one game series that has had me re-playing it again and again. Shadow of Chernobyl I must have played about 5 times, Call of Pripyat the same, and Clear Sky twice. I just sort of get through them and then the desire to play again comes back - absolutely my favourite games
The endless and amazing quality of some of the mods available make it even more likely you will get good replay value from them, though of course they may not be everyone's cup of tea.
That said I think maybe only SoC is actually THQ - the others might be GSC game world ... I get confused with publisher / developer ...