As of today, many games do not take advantage of Hyper Threading. They'l run just fine on 2 or 3 physical cores. However, in the future, the games are bound to become more CPU intensive. In such a scenario, they will benefit immensely from Hyper Threading. Hence, HT enabled processors like i7 series are better buys, for the long term.
In any case, even today, turning off Hyper Threading won't have any significant effect on the game in terms of load time or fps. It'l run neither better nor worse. All the top end games like Crysis, COD MW II, GTA IV can utilize upto 3 cores, with a small performance boost from the 4th physical core (if any).
"How do you propose to turn multi-threading off?"
You can do it from the bios.
Hyperthreading, yes. Multithreading, no. Unless you manually disable CPU cores.