taking a chance you'll come back here after a two days without a responce and going to chime in (just saw this thread)
Everything is substandard in the sense that its a dead end upgrade. First and foremost AMD64 processors were overthrown by intel Core 2 Duos. The lowest model Core 2 Duo (E6300) will be quite a bit better than an X2 3800+. Your motherboard choices for Core 2 Duo are 965, 975X and nVidia 680i based. In the case of 975X double check that it does indeed support Core 2 processors as older models did not. People have not been having the best luck with the 680i based motherboards (few bugs and issues, some bad) so your best bet would be one of the intel chipsets if you went down the intel route for the most painless experiance.
Now theres nothing wrong with going the AMD route either, especially if its considerably cheaper, but the other problem is that you've basically listed components to a dead chipset. AMD is no longer releasing new parts for Socket 939. AMDs newest socket is AM2 which also uses new processors sold under the same name. AM2 X2 3800+ for example.
In both the Core 2 Duo and AM2 cases they also require DDR2 memory, what you have been looking at for that board is DDR1. DDR2 tends to be a little more expensive, but if you dont plan on doing any serious overclocking, you should be able to find some DDR2 667 at a reasonable price. That memory speed (667) is the default for most motherboards of either AMDs AM2 or Core 2 Duo so you wont lose anything by using it. Some people consider it the lower end of DDR2, but its perfectly fine.
The graphics card, i highly recommend against any Geforce 6/7 nVidia cards for personal IQ and driver performance reasons. An 8800GTS, X1900XT, X1950XT or X1950 Pro are all good options and should be available for you to look at, as well as being considerably faster than a 7800GTX. Get what ever fits in your budget, but i personally wouldnt spend more than 270-290Euro on any of those parts. The X1950 Pro is considered the best bang for buck card at the moment by many people here so thats problably your best bet. You can get something more higher up the foodchain like an XT or especially a GTS (GTS is also DX10 compliant and the only one of the bunch that is), but i just wouldnt break that 270-290Euro mark. Not worth it since supply should be ample and demand should be low. If you're paying more you're getting cheated a little.
Sound card is fine, nothing wrong with X-Fi, or Audigy 2s for that matter.
Just so you know as well, the graphics cards are about 22-23cm long, make sure they can comfortably fit in your case beforehand.
