I have an X2 3800 + in my machine right now (with 1 gb dual channel DDR and a 6600GT PCI-e video card) and I love it. I built it in January and I don't question my decision at all ... it was money well spent at the time.
Now, though, I don't know ... AM2 around the corner, using DDR2, and the new Intel, also DDR2 ... like other posters said, DDR2 is not backward compatible and won't work in an DDR motherboard.
I think your suggestion of Intel 805D is a good one. Look at it this way: the cpu is very cheap (around $130) and the compatible motherboard is cheap (around $90, right). And you can reuse you Linux with no problems (reusing OEM Windows XP might be problematic if change motherboard, but there's ways around that :lol: )
Get the 805D and inexpensive motherboard now. Put your money in the best ram, video card and hard drive you can afford. And go ahead and buy a 500-watt power supply if you can afford it.
If the 805D turns out to give less performance than you'd hoped, you'll be set up (with good, fast memory and big-enough power supply) that you can overclock it up to 3.6 territory (4.1 ghz like in that article is not a guarantee for every chip).
Six to eight months from now, you can take another look and see the reality (around January or February): Is Conroe as good and inexpensive as promised? Is AM2 better than expected? Did AMD ramp up to 65nm production faster than some predicted? What are those new 65nm AMD AM2 chips doing compared to Conroe?
Then you can choose either Conroe or AM2. Buy new board, new cpu, reuse all your other stuff.
Also, don't believe the hype about buying a Conroe system in July. Other articles have indicated (including here on Tom's) that the first Conroe systems from OEMs (Dell, etc.) probably won't be WIDELY available until late August or early September (just in time for the back-to-school shopping rush, new computers for college kids).
See, Intel is going to treat its big customers right, first -- especially Dell, which has suffered somewhat due to Intel falling behind in the performance race. Sure, Intel has lots of fabs and is ramping up production of its new Conroe (Core 2 Duo), but Dell is going to get the lion's share first, followed by Sony, HP, Apple, and all the rest of the big boys. They're going to grab up every single chip from Intel as fast as Intel can get them into shipping trays.
The fanboys expecting to go online and buy a new Core 2 Duo (Conroe) chip in July are deluding themselves, IMHO. Maybe August, but it could be September (!) before home builders can depend on getting their hands on some.
Anyway, that's my analysis from reading (with a critical and discerning eye) all the stuff that's been floating around for a few weeks now.