Given your current specs and the fact that you're not running anything except the games, turning off hyperthreading is virtually useless.
The hyperthreading feature was intended by Intel for its CPUs to handle multitasking functions - that is if you're playing games and listening to mp3s or watching DVDs at the same time. If you're only playing a game and nothing else, HT is just another useless program.
If you really want to see some gaming improvements you could opt for better hardware -- 3 GHz CPU, 1GB DDR400+ memory, Over 100GB HD with SATA, Radeon 9800 PRO 256MB or GeForce FX 5950. But overall your system looks great and I wouldn't recommend an upgrade until maybe next year.
Hope that helped