im not recommending he buy an 88gt. im suggesting he use it as a minimum requirement to consider. games like bf3 are using it as minimum requirement so i cant in good faith recommend a lesser power card. thus the gts250 or 5750 both of which are slightly more powerful.
as the the 5850 and 5870 crosfire its the first i have ever heard of it. so could you link that please. i like to keep up to date on these things
.. personally i wouldn't recommend it as you will gimp the more powerful card by a good margin... anyway if you look around you can find xfx 5870s vapor x editions for £150 and you can get a second hand 1 from the overclockers club for less than 100 from the rite seller.
edit* actualy i found an article on crosfiring mixed cards.. with benches... gotta say im surprised that it even works but the thing that didnt surprise me is that its not as good as crossfiring 2 5850's so not only do you loose out th 15 or so percent from having a 5870 you also loose another 2 or so percent because of the mix and match...
so good if you on a budget but not so much if you can find another matching card for a low price.
but thanx for the heads up. i learned somehting today