Due to Microsoft VISTA release I think we have to move there. It means apparently a new computer. I have planned to use a ASUSTEK  Commando motherboard. Ok. But the question arising is: how much RAM can VISTA or any applications actually use is it 2 or 3 or 4 Gbyte? My question is in order not to spend more than necessary on memory.
As long as the 32 bit OS cannot use the memory it must be a bad investment, right?
Anybody got an idea??? ( 64 bit can use 4 Gbyte RAM right? ).

Most often recommended for Vista is 2 GB of RAM and a minimum of 256 MB RAM on your video card.
The 32 bit Windows 2000 and XP can both support 4 GB of RAM.
The 64 bit Windows Vista Home Basic can support 8 GB, Home Premium 16 GB, Business, Enterprise and Ultimate 128 GB.
Most motherboards for home use now support up to 8 GB.
If you want to use all the bells and whistles in Vista, I would say 2 GB at least. I wouldn't try to run it with less than 1 GB, unless you are prepared to limp instead of run.

Ok, thank you. I have planned to go for 4GB. Lets see what it will come of it.
Any suggestion of a video card, Matrox? other see below?, mostly office and WEB related work, not much gaming.
Here it could be:
Asustek Commando
Intel Core 2 Duo E6600 ( I like Intel not AMD, my choice ).
Ki, 2048MB KiT, 1024+1024MB 667MHz DDR2-RAM DIMM, 4GB ( or 800 if applic & availab)
Seagate Barracuda SerialATAII 7200.10 320.0GB 16MB , 4 pcs
Club-3D GF 7800GS 256M tv A card
Zahlman cooler 4 pcs
+some bits & pcs.

On the memory, most of the reviews I have seen say that 667 mhz is the better choice. The performance increase with 800 mhz is not great enough to offset the price difference involved.
Most of the core 2 duo boards are set up to use PCI Express video cards.
Note:
Using a serial ATA hard drive requires entering a special driver during the Windows installation for all of the boards I have seen reviewed since that bus is controlled by a different chipset than the IDE bus.

Hello, thank you for your answer.

Have you seen: http://msdn2.microsoft.com/en-gb/library/aa366778.aspx

there I found some info on this issue.