I think Counter Strike 1.6 is probably still the most popular (not sure if that's true anymore). The most popular games are always the least demanding.
I think Cryis will be the Quake of this period in terms of moding. Back then only people with fancy "pentium" PC's could play Quake. I remember I couldn't play Quake on my 486
Right now the 8800GT is at a good price...a couple of years from now "low end" players will be playing Crysis mods.