Doom 3 and HL2 will run completely fine on a Celeron 2.6, it just needs the video card
most people who buy Celerons also get crap video cards
most people who buy Celerons also don't know much about computers (not referring to all people). they don't know what a video card is, so when they try and run Doom 3 or something and it lags, all they know is the "CPU" is slow
people like that often refer to the whole system as the "CPU"
in reality, Celerons are not much less than Pentium 4's in terms of performance. people just have some huge hate for CPU's that get even slightly beaten by another
*edit*
TF is run on a 2.6 Celeron.