I have a few friends currently doing their postgrad studies in fine art at the various london schools and the impression i got was that none of the them seemed to have met anyone from oxford on their respective courses at Chelsea, the RA, the RCA etc. On top of which, there are virtually no well known contemporary artists who studied at the Ruskin. You could say it's a result of their size but slade students (the slade takes 40 a year whereas the ruskin takes 25) seem to be as present on the british art scene as anyone else. Not that the Turner prize is the be all and end all of contemporary art but, as some one mentioned before, if you went through the cv's of the various nominees you'd find that a lot, if not most, of them went to the Glasgow School of Art, Chelsea, St Martin's, Goldsmith's or the Slade for their first degree. I think most people (including myself at some point) sold themselves on oxford because oxford is oxford which is fine, just bear in mind that there are several factors indicating that it may not be the best place in the UK to study art.
PS. I recently met a guy who'd studied history at oxford for eight years and he was under the impression that the Ruskin wasn't even part of oxford. Just saying
PPS. RE: That_Diesel. You should probably heed your fellow artists' advice. You have A LOT of work to do before you can hope to get into the ruskin or any fine art BA course at all for that matter. I'd suggest doing a foundation first if you're serious about art.