2ghz Core 2 processor, shipped with 2gig ram & Intel x3100.
K, 1st thing 1st. Gmod (and all HL/HL2 based stuff) is basically a DX7 engine with additions for newer tech bolted on top. In essence a 6 year old computer would be capable of running it.
TB however has a lot of maths going on constantly, thus it needs a bigger machine.
To sort your problem I advise the following:
1. If you've got shaders enabled turn them off. If you can't live without them turn off most the shader options. Your GFX chip emulates shaders so your giving your processor more work by having them turned on.
2. Set the game to 30fps in options, although with shaders turned off your machine should be able to do 60.
There is an annoying glitch with the recent versions - when the game finishes downloading your items & textures the fps will half. It's completely random and I haven't found the cause yet. When that happens & you exit the game there's a chance it's still running in the background, open the task manager to kill it - it'll be listed as a process rather than application.