You have to definately get 2.4 Gig Hz phones. Just about everyone has one at BestBuy, and now I think I saw a few 5.2 (?)Gig Hz phones at some place.
I've got a 900Mhz GE, and it is crap; for the business line. The rest of the house has 2.4Ghz.
Make sure you get full-duplex as well; not half-duplex. Full means your handset can talk and listen at the same time. Half means your handset, can do one or the other; have to take turns talking in conversation.