You can check this site HERE
at tomshardware.com to pit different processors against one another. The link should bring to you the AMD 64 3800+ agianst the Pentium D 805.
My recommendation is to save a bit more and get the core2 duo (intel), the site you provided lists the E6300 (1.86ghz). On the chart that I linked to, the E6400 (2.13ghz, which is the slowest core2 cpu that this chart lists) is a lot faster than the Pentium D 805 (see chart HERE
Granted, it's more than you want to spend, but you'll get more speed out of it and it'll save you from having to upgrade in the future, too. The core2 is also able to overclock well, too, even on a stock cooling fan.
If the core2 is out of the question, you can use that chart that I linked to to see different results for benchmarks (games, processing images, etc.etc.).
Hope this helps!
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