Source engines are only single threaded so it needs raw clock speed and an efficient architecture. Moving from yours to even an E2160 @ 3GHz would be a large step up. My old AMD X2 was limiting even my 3870.
Something still doesn't add up. Althought the Pent. D is crap(slow), still doesn't explain the lack of gpu usage. I still have a Pent. D 840 and yes it would just make you slow but you should still see 95-100% cpu usage in 3d apps. Try Furmak in a windowed low resolution mode, like 640 x 480. Run gpu-z at same time with furmark in either a benchmark or stability test. See if then don't get close to 100% cpu usage.
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