You said it is old, so I am guessing it is this >>> http://www.engadget.com/products/dell/dimension-xps/gen-4/specs/
. That is.. currently on display at the Computer History Museum, so NO there is no 'upgrade' option here. There is no PCI cards anymore (for like 10 years they not been produced if not longer!!!) so you are WAY out of luck on this.
A simple CHEAP solution, get a i5core desktop from Walmart for only $349, includes mouse, keyboard and sometimes a free LCD.