HT will offer much less, as it's really for multitasking. Even if the game were multithreaded, I doubt it would offer a big boost.
I'm sure Valve optimized the game for SSE2 though, as that has been out for ages..
An interesting question is, how much of a performance boost would going to AMD64 give HL2?
The extra registers would definitely help out alot, plus using SSE2 solely instead of the archaic x87..