i think they no longer have 5970 in stock since the card has been EOLed by AMD. but it doesn't mean they can give you something that perform worse than 5970. i think 5970 sit between 7950 and 7870 in term of performance. that is if CF working properly. if not it perform like a single 5870. if they can't really give you any better than that then you have to make do with 7870. just OC the card.
personally i'd take an OCed 7870 over 5970 any day. but that's just me though :P
What about the 6970? Does 7870 perform better than 6970?