may change from XP to Win 7, but which version should I buy ? also with Win 7 64 bit, can I still run older 32 bit programs ?
If you're buying Windows, you might consider waiting until Win8.1 comes out (October, I think). Be sure you have a 64-bit processor before buying Win7 (64-bit), ortherwise it won't work. Plan on any 16-bit apps you still have not running on Win7 or later even under emulation (e.g. VMWare Player).
Get 64-bit so you can have more than 3.2 GB of RAM. The system will probably be too slow if it has 3.2 GB or less of RAM.
Windows 7 Pro 64-bit ... it has a XP compatibility mode (additional download) which was needed for me to run one really outdated app, as well as the driver for my HP scanner (HP seems to be good at that, ie. not updating their drivers to force their install base to buy new hardware).
Microsoft has a tool which you can download. It will tell you if your system is Win7 compatible. I suggest Win7 Ultimate x64. You can run almost all x32 programs on it but if you have trading specific software, ask the vendor if it's Win7 compatible. http://www.microsoft.com/en-us/download/details.aspx?id=20
I have been running Win7 Home Premium (64-bit) since 1/2010, and I have yet to find a 32-bit program that won't run. I think buying Win 7 Pro or Ultimate is a waste of money. If you need it, you can easily buy an upgrade. Every copy of Win7 has every capability. The upgrade just unlocks the additional features.
Likely, yes. W7-64 has a 32-bit emulator. Check your Task Manager. If your fully loaded trading setup uses 3GB-ish or more RAM, you should use 64-bit. If it uses 1-2GB RAM, you can use 32-bit with no negative consequences. There are still some 64-bit issues which can be avoided by using 32-bit. BTW... I use 32-bit. No intention to change to 64-bit.