Assuming it uses a slot 1 processor, you'd be hard pressed to find slot 1 processors, and without knowing the exact cpu in there, there might not be an economical choice. If its using a 200-300mhz slot 1 pentium, then its hard to say if you can find a slot 1 available in higher speeds. In addition, you're only adding some megahertz which would increase performance by less than 10%.
You could possibly install a slot 1 to socket 370 adapter. Socket 370 processors would be easier to find.
You probably have an agp slot that would accomodate a better graphics card, but again, we're only looking at a few percent increase in performance.
The best you could hope for is running a very slim Windows XP installation and using it for email and web browsing. With all the new features found in web design, even this seemingly simple task may run pretty slow.
I don't use Linux much other than to play around and experiment with but you may find reason to go Linux because its completely free and you probably wouldn't want to dump any money into this unit.