The beachball in itself isn't the end of the world - it normally just means the frontmost application (or rather, the application whose window you have the cursor over) is busy doing some time-consuming task without starting a separate thread for it.
Earlier versions of OSX (particularly, 10.0 to 10.2) were very prone to this, but much less so with 10.3 and 10.4. Having insufficient RAM can be a cause of this - I wouldn't buy any Mac with 512MB (or less) of RAM these days, and certainly not a MacBook, where Rosetta and the shared VRAM are going to use even more of your already scarce free memory.
I think a MacBook with 1GB of RAM would be a fine buy.