I would also run a few extra AntiVirus scans.  It could be a virus or worm.  Also, it's always better and faster to rely more on hardware than software for graphics.  Software based solutions use up too many CPU cycles to be efficient.  With Carrara as with any other 3D program, the Video subsystem is pretty much the most important thing next to a fast CPU.  The AMD Quad Core CPU I use is faster than heck with Carrara.  Noticeably faster than my old Dual Core system.  Three quarters of the annoying memory allocation errors, as well as the unknown error errors have disappeared since I got this system put together.  I use the same graphics card you are thinking of.  It's a great chipset.  And ever since AMD took over ATI, their CPUs have been much faster when paired with a good ATI card.  I recommend the Sapphire Radeon HD 3650 card with the 512MB of RAM.  I compared a bunch of brands of that particular chipset and the Sapphire one was the fastest in benchmarks.  That's why I picked the Sapphire and I have been nothing but happy with it ever since.  Like I said though, steer away from Name Brands, get a clone.  Ever since the floor fell out of the economy, most of the name brands have switched to using cheaper and usually lower quality components to keep their profit margins up.  Good luck.

Rich

P.S.  The AMD Phenom X4 is a CPU, not a graphics processor.  And you do need a multicore CPU to take advantage of multithreading.  Multithreading just lets the software use those multiple cores.