Do you want to just display the same image on both monitors? http://www.google.com/products/catalog?q=vga+splitter&rls=com.microsoft:en-us&oe=UTF-8&startIndex=&startPage=1&um=1&ie=UTF-8&tbm=shop&cid=14064807061771269302&os=reviews
If you use a splitter, you need to make sure both of them can support the same resolution and refresh rate.
If you want to run true dual-displays you need a video card, any lower-end PCIe card should work with the system, if you have the small case, you'll need a small form factor card. Radeon 4250 or something will work.