I've got a 4850 and am more then happy with it It kicks ass. so a 4870 can only be worth it ^^

The only word of warning I would have is about temperatures.. my 4850 (from gigabyte) ran at 81C idle.. I didn't dare testing it at full load :S I have read that It is acctually expected to run as hot as that! :S so I got myself a new cooler and it runs at 60C idle now.. and around 90-95C when under full load.. to be fair that was in our recent heat wave and with all case fans set to minimum, and my PC case in a corner under my desk with the least amount of possible ventialtion.. so I expect it to run a lot cooler over the next few months (it's brand new too ^^ so my thermal paste application needs to get worn in)

I don't know why they are running that high, but just don't freak out when you see the temps ^^ I've heard nVidia's cards run at around 45-50C idle under their stock coolers, so they may be stretched further when OCing imo.
But I hear the 4870s are running cooler then the 4850s so you may be ok