Since you are already familair with Intel the 820 would be the better choice. You're not gonna notice that much difference in gaming between it and AMD's top offering which righ now is the X2. Like was stated earlier, the video card will make more of a difference.
For rendering the 820D is without a doubt the better choice. As you are already aware. Intel systems are much easier to setup and OC. They run smoother, with fewer bugs. When you are doing video editing the last thing you need is a system reboot or freeze.
The top Dual Core bpoard out there right now is the ASUS P5WD2 Premium, and I'm an Abit fan myself, but the Asus is the better board right now. Yes, it takes DD2, but if you want a new system, you're gonna want it to perform better, and bringing over old RAM will only cause a bottleneck in these new systems. It's really no different than it used to be. Replace the mobo, CPU, ram, and graphics card.
With the 820 or 830 on that board and good RAM you will will get great all around performance and will have no issues doing anything at all you want to do, and belive me nothing will be lacking...well, except for instability, becasue you won't be seeing any of that.
These 820's, 830's and 840's are great OC'ers. It is very easy to 4.5GHz on air with one, and 5GHz (2.5/core) is not unheard of.
If you want to step down in price a litle a 6xx CPU is also a great OC'er that has shown absolute stability, and gets high marks.
There are also new CPU's coming out from Intel in the first quarter and second quarter of '06 that are pretty much gonna make everything else look like legacy hardware. If you are wanting to wait a bit and study up on these I would highly recommened gving them a good strong look. I buitl a 478 system about 5-6 months ago now, and have been waiting on these. I have seen many good CPU's and have been tempted to build another one, but these new CPU's are going to be so good, and are goijg to be so much more advanced that they could turn a 2000 dollar system into a 200 dollar one the day they come out.
There are many great choices to choose from as you can see.
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