i have a denmark 170, you might want to invest in a new power supply if you get a dual core. i recommend OCZ check out the link.
http://www.newegg.com/Product/Product.asp?Item=N82E16817341002 because it is dual core you need a dual +5v rail line (2 - 4pins lines, check your current power supply, i dont think you have it) the socket 939 Opterons are not on sale so stay away from them, good processors but you have to remember that 939 is being discontinued so why invest alot of cash. what you should do is OverClock to get the most from your system and starts with a quality mothrboard. But you listed some specific choices so here is my answer..........I would say the 4800 to your original question......for the other question i would go with 4800. But i would choose to go with the 3800 on the cheap. check all around for best prices.....even wait a whole month if you have too. Check out some of these E-tailors.
Neweg.com
ZipZoomFly.com (best prices currently on what your looking for)
atacom.com (never shopped here, please leave any info on your experience if you shopped there before)
tigerdirect.com
monarchcomputer.com
Good Luck and be patient!!!!