Although marked as Solved here is an update on the choice I made.
Given the response and having read the manual and having (still) a positive experience with first installing and running Window XP in Virtualbox on my HP laptop, I was convinced that an i7-2600 processor with 8GB memory a 1gb Nvidia graphic card and 2 x 2 TB disks would give Windows 7 enough resources to run smoothly in a Virtualbox.
After two weeks of running, I have now the below settings for Virtutalbox runing Windows 7 ultimate 64 on PCLinuxOS 2011 using a pae/bfs kernel.
base memory: 3584 MB, chip set: ICH9, enable IO APIC, enable absolute pointing device, 8 cpu's of 16, not enabled PAE/NX, acceleration VTx and Nested Paging enabled.
storage on SATA controller: 3 vdi's (windows drive c, e and f) on different partitions
display 128 MB; 3d and 2d acceleration off
usb enabled, usb EHCI enabled
shared folders: 2 NTFS partitions
I have managed to install Windows 7 with SP1, using Norton Internet Security 2011. I have Visual Studio 2008 up and running. Since Windows requires to choose a browser, I installed Opera as browser (I use Firefox in Linux). I have also installed some of the other main software packages I intended to install. And they all run fine.
That is all fine except for the fact that is was not as smoothly as with WinXP and Window 7 it is not as responsive or as stable as I would like it to be.
Installing Norton Internet Security 2011 required several attempts. Even now that it is installed on my virtual E: drive it does not always start correctly; especially not after Windows updates. You need to do an extra reboot to make it work again. Even then you may get a spontaneous message that the anti virus software is not running. I disabled the PAE/NX option in Virtualbox, because NIS does fail more often when it is on.
With Windows 7 open and nothing in it running and just leaving it, I got, after and hour or more, a spontaneous critical error from Virtualbox. Happened so far twice. I have now turned the options for power control in Windows off as much as I could. I hope that helps.
Windows 7 is sometimes very slow. It not always clear why. Especially with copying files or cleaning up or searches. So, in general with disk operations. After installing SP1 I use cleanmgr.exe to clean up the back up files and all restore points except 1. I got half way and two hours later I killed the job.
The administrator is not always the boss. There is a guy called Trustedinstaller who owns some important folders. Fortunately as administrator I could change ownerships and take control and delete some files the clean-up tool would not.
Shutdown sometimes just halts.
as the manual already indicated; picking up one of the USB devices (a printer, scanner or external disk) makes them unavailable for the host. That does not mean that they work.
The Canon MP830 scanner part does not work; the printer does (could not get it working via a network link to the host printer). Linking an external disk is fine, until you start using it for backing up or restoring or just copying large volumes of data. You only do that once. It is not suitable to use for backups. It is quicker to keep the disk on the host and use the partition linked as shared folder as an intermediate station. I have noticed that you can actually share a folder from a USB hard disk mounted on the host and that has (at least it 'feels' like having) a far better performance.
Turning sound on for the guest turns it off for the host. That is why i set audio for the guest to off. Do not need sound on Windows. The customised sound card links to my HiFi and aTunes, Radiotray, VLC all work fine. Listing now to jazz from a Belgium station which is no available via my cable.
So back to a dual boot? No way. The main reason for that is that with only about 5 or 6 major applications, I only need Windows for very specific tasks. By using the shared folders I can easily exchange files with Linux and with the bidirectional clipboard I can copy whatever I want. Despite some of the issues (which are annoying and sometimes worrying) Virtualbox is still a far better functional experience than a dual boot with far less overhead There is another very big advantage.
I rediscover Linux.
I had e.g. a complex Access database I used for my normal household administration plus few more much simpler databases. With Access already installed on the WinXP guest, MS gave me no activation under Windows 7. Right. For professional reasons I may still need to consider buying Access 2010.
But, I discovered LibreOffice Base and find it is good enough for some of my simpler databases. And then I discovered Skrooge. Oh boy, that I actually missed that one !!
Low learning curve, nice easy csv import, ability to set up own categories. I took me half a day and I have a Linux based financial administration with all my previous Access data since 2006.
The scanner may not work under Windows 7, but it works of course under Linux. But I was used to Paint Shop Pro, because it was so easy to use and quick for processing the scans. Well, I journeyed back to XSane and this time I spent a bit more time in trying and discovering the different options. Once you how on which window to do what, things as actually easy. Just a bit different.
Sorry for the length of this post, but I hope it may be useful for who wants to take the same dive..