yes you can.. but it will work just as if you had crossfired 2x 4870's
so you'll be wasting a lot of power...
in other words....
if 2x 4870 CF is as powerfull as 4870x2
you'll gain no performance over an standalone 4870x2 with an extra 4870...
so.. I think you have got 2 option here.
either wait untill you can get second 4870,
or buy 4870x2 and sell your old 4870
(you can use that money and a little extra to buy another 4870x2 and then CF)
hehe but seriouslly... what's ur PSU
You can't say he'll gain no performance. Cause he will gain some, it just won't be as big as going from one 4870 to 2 4870.
And I second the question, what PSU do you have?