Your entire rig is in need of an overhaul.
the AMD athelon gets the job done, but not as well as an ultra or a phenom. The Ram is severely out of date. That "400" is the bus speed measured by megahertz (Mhz). These days a stock computer has between 800 and 1200 mhz on their ram, and are usually packing DDR2 or 3. The GFX will give you less than optimal game play for any game that utilizes free-roam mechanics.
You will need to at least an upgrade to your GFX... a 9800 GT or GTX (1GB DDR3 or better) should give you the entry-level quality of UM, and if you have less than a 400 Watt power supply, I would suggest upgrading that as well.
Honestly if you can afford it, I'd just do up a whole new rig.