Most newer video cards require at least 2.6 ghz to run without being bottlenecked..:)
That's a 2.6 ghz Core 2 or equivalent cpu....
An athlon 64 x2 4400+ at 2.6 ghz is still going to be lacking. Even if you got it up to 3.6 ghz, a stock phenom II or Core 2/i7 would still kick the pants off it. On the other hand, that magic 2.6 ghz figure is referring to high end graphics cards (4870/gtx 260 and up), not the 4670. Overclocking would still give you better performance, so give it a shot. Like coozie said, just don't expect miracles.