If you have a PCIe 1.0 x 16 or PCIe 2.0 x8 for both Xfire slots, you should be fine with 2x4890s.
2x4890s would perform great and be better than a 5870. You can get a 4890 used for about $150-$160ish.
you have a good point sir but he will be stuck with pretty much old tech and the next upgrade he will probably be taking is 3 years from now. further more power hungry cf
set up and ati cf driver problems could be stumbling blocks down the road... just my 2 cents