I am assuming that the game has been patched?If not get it here: Patch
Found here: Tweak Guide in it says: Fast CPU and Fast Graphics Card: People are complaining about how difficult it is to get playable framerates in T:DS, but upon closer examination their systems reveal a weak CPU or a slow graphics card which explains why this is so. As with most recent games, especially games based on the Unreal Engine (like UT2004 and America's Army), T:DS requires a balance of both a medium to fast CPU and a medium to fast graphics card for optimal performance. If you have a fast CPU but a slow graphics card for example (e.g. P4 3.2Ghz combined with an FX5200), or vice versa, then your machine will struggle. The CPU in particular comes under heavy load in T:DS because of the steep AI (Artificial Intelligence), Physics and rendering requirements, while the graphics card shares the load by undertaking advanced lighting, shadows and pixel shader effects. While you can reduce the load by reducing the in-game settings and undertaking some tweaking, you cannot turn off the advanced lighting effects, tone down the physics or reduce the AI hence the steep requirements.
You have not got a particularly fast CPU or graphics card, though you fail to mention the exact model only what series which is 4800. You might find it will run better after your proposed upgrades.