I am buying a new computer with Windows 7 Professional installed. I need to install XP on the virtual drive because I am running a program (AutoCAD) that I do not have the funds at this time to upgrade to Windows 7. The computer I am buying will have 6GB of RAM. Is that enough?
Firstly, what makes you think that AutoCAD won't work natively in Windows 7? This page implies that whilst is isn't supported, it still works. Assuming you mean 6GB of RAM, that should be more than sufficient.
I have Windows 7 and run programs designed for XP (and in fact for Win95). They work.
I've had one that was a bit dodgy. I changed the compatibility setting for that program (located the EXE file, right click, select properties, change the compatibility value) and now it works just fine.
Microsoft go to a huge amount of effort to make sure that windows contains compatibility hacks to work with programs (often containing bugs) written for previous versions of windows. I have had no reason at all to use XP Mode. It all just works.
On the other hand, if you are using older 16 bit software or DOS programs that worked OK in XP, you will have trouble in Windows 7, 64-bit edition. That's because the 64 bit OS will run 64 and 32 bit programs, but not 16 bit programs.