As I understand it, the definitive study was from New England Journal Medical from June 30, 2005 (don't have web reference for this), done with about 2000 patients taking either 5FU or Xeloda. The study basically indicated that Xeloda was as effective as the IV form. The results can be seen on around page 5 of http://www.rocheusa.com/products/xeloda/pi.pdf
, which is the manufacturer's information on xeloda.
These studies only compare 5FU and Xeloda, they do not compare 5FU and oxilplatin and Xeloda and oxilplatin, so its not been proven if there would interactive side effects. I believe there are studies going on in this area, but I don't have the reference.