Windows XP is taking up a good chunk of your RAM, so depending on cost, I would look at upgrading that. I think the general rule of thumb for MS operating systems is to double the minimum recommended RAM to get something acceptable.
I had no problems with a Duron 1.3GHz, other than a couple 1 second slowdowns. Unless you are doing other CPU intensive things or find a great deal, there is no need to upgrade your CPU.
You should be able to find a better video card that is relatively cheap. I would try upgrading the RAM first, though (depending on your budget) since that should help with the system in general and not just with games.
Do you have an LCD monitor? Just curious; most of them are set to a 60Hz refresh rate. If you go into your video driver options there may be an option to disable vsync (mine is under a 3D tab of the Display properties, in the Direct 3D options). Vsync basically tells the computer to complete one full frame before starting the next. Disabling it can improve the frame rate in games, but may lead to a lower image quality.
For Beyond Divinity I don't think you need to worry about it; even 60fps should be good enough. I tried the demo on a laptop that had vsync enabled and 60fps was fine. The only reason to want frame rates as high as possible is so the game doesn't slow down when there are a large number of things on screen, but this is much more of an issue in first person shooters and flight simulators.
The speed of you hard drive would have an impact on load times, but probably not the main effect. Unless you have a 5400 rpm model and are doing other things involving a lot of slow loading, I would not upgrade/add a hard drive.
