You'll want to leave the HT on, for sure
Mafia 2 never gets great GPU usage, because the load from physX calcs doesn't show up in your gpu usage % unless the card is dedicated to physX only.
GTAIV is very cpu-intensive but ttbomk can actually only use up to 3 cores, and only 2 of them fully. Thus, if you're steady around 70-75% usage on the CPU in that game, with a 4 'core' cpu (or dual core w/HT), then you could well be getting CPU-BN'd.
People around here often try to convince others that they don't have CPU BN unless they are seeing 100% cpu usage, but that is actually pure hogwash. It's much more complicated than that.
A lack of 100% cpu usage is actually only a reliable indicator against CPU BN when you have a single core CPU. Once you have, say 4 cores (be those physical or logical CPU's i.e. via HT), it becomes possible to become BN'd at 25% of CPU usage, if you're playing a game that's only coded for one core, 50% if it's a two-core game, etc.
Lastly, just so you know ... the number of cores that you see 'activity' on in Performance graph of Task Manager doesn't actually tell you how many 'cores' the game is coded to run on.