Hi,
5870 can be found about anywhere. As for the nVidia counterpart, it gets launched this Friday with 2 models GTX470 and GTX480 based on the Fermi architecture. We don't know what availability will look like, most tend to agree that it's is going to be hard to get it. Yet again some other rumors say that the amount will be high at launch time. also, some other think that there won't be any available on the normal channels until April 6th. So, basically, next month should be very interesting to build your new rig, but we cannot tell you what would be the best choice right away.