Phenom X3 8450 and Athlon X2 6000+ cost the same ,I do not know which one to buy.

AMD Phenom X3 8450 Triple Core Processor
(2.1GHz, 2MB, 1800MHz FSB, Socket AM2+)

AMD Athlon 64 X2 6000+ Duel Core Processor
(3.00 GHz, 2MB, Socket AM2)

Help me choose please.

It depends what you're using it for.

In almost all cases the 6000+ will do better. The X3 will benefit from the extra core in some situations, like picture/video rendering and a handful of games.

Personally I'd be looking at spending a bit more and getting a top quad like the 9950 black edition. At the very least a faster 3 core like the 8850 at 2.5GHz.

Here's a comparison of the 8450 to the 6400+ and 6000+. The 6000+ beats it in everything except Supreme Commander, video and 3D rendering. It must be painful for the AMD techs to see results like this from it's newer products.

Here's a copy/paste from a review:

The three cores simply aren't quick enough, individually, to make this triple-core product look appealing. They're a liability in single- and dual-threaded tasks, where the X3 8750 sometimes falls behind the much older Athlon 64 X2. The X3 8450 almost always does so.

From here.