Yeah, I agree with car avvie dude... no need unless you really want to waste you money.. if you're thinking of an upgrade, might as well wait for 1333MHZ FSB chips to come out for Intel mid next year, even though there won't be that much more of a performance increase either...
Gaming wise, you won't see much more performance increase, not enough to merit a new CPU. Video decoding... is that like video encoding... I mean, are you just watching DVD's or are you ripping DVD's and encoding them in H.264 or XVid format?... if you're encoding, then you'll notice a fairly major difference, but otherwise, your comp is fine.
The E6600 would be faster, however the potential of your X2 3800+ isn't even being used yet so I wouldn't spend the money on the new CPU. If you HAD to buy a new CPU, the E6600 would be better than the Opteron 170.
The 3800 X2 is fast enough to not warrent an upgrade on your current platform, but too fast to be worth switching your entire rig for. Stick with it, or like prozac says get into overclocking. It's better than...well...it's better than a stock CPU.
(btw Prozac, thanks for that link to the site RE: Kentsfield compatibility)
The Opteron 170 is basically the same thing with more cache ram.
I love mine, but I built before C2D or AM2 came out. You can find s939 Opterons for a decent price these days if you want to make an incremental upgrade, but don't waste too much cash on old tech.
I too have a X2 3800+ but I am overclocking it at 2500MHz, I am also looking at getting an Opteron 170, my 3800 is just not doing it for me and the opterons can be pushed much futher (or so I've seen) If you arent going to overclock then the performance difference between the 2 AMDs will not be that great, The e6600 on the other hand will out do the 170 at stock speeds but that means that you would need new RAM & Motherboard