HL2 used stock Source engine.
HL2 EP1 was capable of using upgraded Source engine which has HDR.
The Xbox 360 version sports these improvements on Half Life 2, but the PC version has yet to see these updates.
Lots of confusion in this thread. The OP is wondering if the improvements to HL2 seen in Episode 2 will come to the original Half Life 2 as well as Episode 1.
If you look at Episode 2, there are better particle effects (for example, blood splatters and fires), dynamic lighting, more reactive AI and so on. The Xbox 360 version sports these improvements on Half Life 2, but the PC version has yet to see these updates.