Dell has always relied on Intel chips for its hardware and the main reason for that is simple: supply. Intel has the facilities to produce the kinds of quantities Dell is looking for. AMD has not, but what AMD has today is something Dell doesn't and desperately needs. That is mindshare. AMD Athlon and Opteron chips are fast and built for running servers and workstations.
I believe they already did once buying Alienware.
http://www.itbusiness.ca/it/client/en/home/News.asp?id=38970