I thought about a similar build (intel) at first, but decided to go with year 2009 instead, to cover all of the XP games. Not an AMD guru, but recently Phil was tinkering with something similar year-wise on his youtube channel.

My ultimate XP build is Asus P5Q, C2D e7600 or one of the higher clocked Core 2 Quads, ATI4870/5850/5870 and Sound Blaster X-Fi for sound - might be an overkill and right on the edge of period correctness.

Quad FX is end of year... if you want best of the best (2x FX-74, nForce 680a, etc.).
Other than that, you can't even have Athlon64 x2 6000+ because it's from Feb. '07...
Sadly, best you get from AMD at the end of 2006 without doing stuff like Quad FX, is Athlon64 x2 5600+ (90nm, 2MB cache version), or FX-62 (same base frequency 2,8GHz, however FX-62 is unlocked).

Top end 939 gear is not cheap. Any FX chip will run you a pretty penny.

I think a Sandiego Ahtlon 4000+ machine would be fun to build. They easily overclock past 3.0ghz, which was bonkers fast for the time. As others have mentioned, the 939 dual cores were the creme de la creme of the day, but they didn't benefit games much. The dual threaded games were dominated by the core2s, and a better choice for that era.

On the GPU side, the 7900 GTX was king of gaming at the time. If you jump forward to 2007 though, the core2duos and the geforce 8 series crap all over it.

I understand why you are going for 2006, the pre Core2 era is interesting, but 2006 is a rough target. It's very early in an era of gaming I tend to classify as 2005-2010, which is the late XP early Win7 gaming era of PCs catching up to and eventually surpassing the XBOX 360. Core2/Phenom and up will basically destroy pretty much anything the Socket 939 has to offer excepting the most expensive FX chips. To make your build worthwhile, I would advise you to find a decent FX processor, but good luck with that. Otherwise, your target build is pretty short lived, and in that case do not attempt to play ports from later in the XBOX 360's life cycle, as most GPU features from that console hadn't properly made its way onto the PC GPU market by then.

For the fastest videocard at the time you want the GeForce 8800 GTX or GTS, good luck finding one alive (Google Bumpgate).

If you are interested in crossfire, by all means go for dual X1950PRO as they have internal crossfire (first card to have it) and can do it without the bridge cable. The fastest GPU ATI had at the time was the X1950XTX (what that stupid model name) followed by the X1900XT and X1950PRO/X1900GT.

The sempron 145 ( am3) is able to cover XP era games until 2005? I dont remember but games started to use 2 core in 2005/6?

An AM2+ motherboard with Phenom II X6 125W support IMO is the best:

1. Give the most CPU options from 2006 K8 to 2010 Thuban. AM2 chips are also generally also cheaper than S939 counterparts.
2. DDR2 is denser, faster and less sensitive to RAM timings vs S939 DDR1 which tops out at only 1GB per DIMM. Also AMD-only 4GB DDR2 DIMM support.
3. Modern quality of life features like 1gbps LAN and AHCI.
4. Comfortably built after the bad caps era.
5. Not as bloody common as LGA775 and immune to meltdown/spectre security holes.

Well I have. a decent base i guess Athlon x64 black edition made in 2005 , however i dont know why but i cant start PC with all ram slots occupied , I can only use 3 and all ramsticks are ok

Socket 939 does support ECC DDR memory, so in some boards you can have 8-16GB of RAM.
Nothing is immune to Spectre, that is why it's such a big deal.
I base that on Microsoft Win10 AMD patch that was rolled out for F revision Athlon64 (AM2) : LINK.
I'm pretty sure you can have 1Gbit LAN on Socket 939 boards.

I think Athlon 64 x2 Black Edition lineup, was created in 2007.
For better stability, decrease TAM speed and increase CPU-NB voltage.

From my testing ive found that WIndows XP can be more versatile that most use it for , For instance there are some late 2000s early 2010s games that will run on XP aka Dungeon siege III and Skyrim (original version) that run best on at least 4 cores ,that being said i would say 2005 and older should be fine for a single core