Well, the 9750/9850/9950 are basically the same performance as a Q6600. They are still good buys at their current price.
With a 790GX board (or 790FX with 750 southbridge), the 9850 BE and the 9950 BE should be able to attain 3.3 to 3.4 Ghz on a good air cooler. My 9850 does 3.4 dead stable on a very easy multiplier overclock and 1.45V. At 3.4 Ghz my 9850 BE outperforms my Phenom II 940 @ 3 Ghz by a small margin, and would estimate that a 9850 or 9950 at 3.3 Ghz would give you the same performance as a Phenom II 940 at stock. That said, Phenom II has roughly a clock per clock performance advantage over the late model Phenom I's by about 10%. You can push the Phenom II to 3.6 Ghz very easily... and quite a few are pushing it to 3.9 and above.
As to whether or not you want to sell your E8400 and go for the AMD quad, it really depends what you are after. In single threaded applications and games, the E8400 will beat out the 9950. Once you get into the multi-threaded stuff, the 9950 really does pull out ahead. These days, with more and more games and apps becoming multi-threaded, it looks like the quad is quite a bit more future-proof.
Now, you can buy a Phenom II 940 + 790GX motherboard on Newegg for $295 total combo deal. A 9850 + 790 GX combo deal comes out to $228. A Phenom II 940 would run circles around the E8400 the large part of the time.
i wont be gettin afermarket cooler till may so i dont oc.. what the multi-threading stuff? i just mostly do basic stuff on the comp. and now and then play a heavy game so...?
Multi-threading just means that the program (game or app) has been programmed to have multiple channels of calculations going at once. A game can have a tremendous amount of calculations that are not very dependent upon each other, and often run concurrently.
For example... Vanguard: Saga of Heroes. A relatively obscure title, but it's an MMORPG offered by Sony. If I remember correctly, they modified the UT3 game engine to offer much better graphics and sound quality. What they found was their new engine bogged down the CPU way too much. So they decided to give an option to offload all the audio for the game to a separate thread, and frame rates increased by quite a bit.
How did it do this? Well, windows can put different threads into different CPU cores. So, while one core is working on feeding the video card pretty graphics to render, the other can work on the audio. This allows both tasks to run faster.
These days, programmers are really starting to get busy about multi-threading games and other applications, to make use of all the extra power those extra cores in the CPU have. Some games currently use more than 4 cores.... Microsoft's FSX is extremely multi-threaded and CPU dependant. Grand Theft Auto IV, while it needs serious optimization, is heavily multi-threaded as well.
So, getting a quad core CPU right now will help you more down the road than a dual core. Plenty of titles out there with 3+ threads right now as it is.
There is practically nothing out there that falls into the category of things you use your computer that a quad is going to do better than what you already have. If you are intent on a quad I agree with nsimo86. It is almost a waste though unless you play of of the 5 or 6 games that exist that take advantage of the quads.