Tri fire with different memory

hi guys, had another thread up but didnt get many responses

im running this system:
Asus p5e Deluxe
Q9450 266@ 3.2 ghz
4 gig DDr2 1066
zalman 800w psu
and a 512mb 4870

Im thinking about buying a 4870 and running it in tri fire, will this be compatable with a 4870x2?
I realise it would be easier with a 1ghz card but will it work with the 512 and what will be the
performance ramifications.

the alternate is to get rid of the 4870 and get a 5870 when they are released. will this bottleneck my current system

any comments would be appreciated
thanks lawrie