I had to look at the dates of these posts, as everyone is really talking about old chips here.
All this really depends on what you want to do and how much you have to spend. If your primary thing is gaming, then I'd suggest the E8400/E8500/E8600 series. These are the newest architecture for Core 2 Duo processors (probably the last of the socket 775 processors), and are about 10% faster clock for clock than the older E6400 etc series. In other words, an E8400 at 3.0Ghz stock will be about 10% faster than an E6850 at 3.0Ghz stock speed.
If you do a lot of multitasking, video / audio encoding, Linux kernel compiles, etc you'll probably be best served by more cores. Your best bet in low priced quad cores is pretty clearly the Phenom II 920 / 940. The triple core 710 / 720 are also very good as a compromise spot. Again, this depends even more specifically on what kind of software you use. For example, if you do all your encoding on iTunes, the E8xxx series is better than most quad cores because iTunes isn't heavily threaded.
I only see the Athlon X2 series being competitive at the bargain basement price points - ie if you want a $30 - $70 CPU, Athlon X2 is probably your best bet. At this price point, only the Intel E2xxx series is available and the Athlon X2's will usually handily beat them.
All this is assuming no overclocking. I wouldn't recommend overclocking to anyone, despite all the hype you see on these forums about it.