Get the AMD. I run an Intel D820 and I wouldn't EVER go below that. AMD's X2's are great processors compared to the Intel's. You can always look into buying a combo processor/motherboard/RAM from places like www.monarchcomputers.com
. That way it's easier to choose what will work with what and you'll get a good deal. You'll also be able to compare different brands and prices much easier.
And yes, you can overclock the 805, but it'll require some pretty good cooling action, which will run your costs up more and still won't be as good an an AMD in some cases.