Help upgrading new system

Hey guys im thinking of upgrading my GPU from my old 7600GT and was hoping u guys could lend me some advice...
I am limited to ordering from overclockers.co.uk since not many reliable sites deliver to Malta.

My other specs:
CPU: Q6700 @ 2.7Ghz
RAM: 4Gb (2x2Gb) DDR2
OS: Windows 7 64bit
PSU: (also changing see below)
Mobo: 4Core1600P35-WiFi from ASROCK

Ok now i'm leaning to the 4870 or the 4890....now my dilemma is that down the line i was going to Xfire the card with another 1 now the thing is when i looked at benchmarks i saw that the 4870x2 ranked very high up on lists whereas the 4890x2 was not even seen on the charts...so is the 4890 not good when Xfired??? as if the 4870 is better Xfired then i guess i should go for that no???

My next dilemma is that my PSU needs changing as it can barely run 1 of these GPUs let alone 2 lol
A friend recommended at least a 700W PSU...and i found this:
http://www.overclockers.co.uk/showproduct.php?prodid=CA...
now the thing is it says that its SLI certified not Xfire....does this mean that it will not run them??
If you can please suggest a good PSU doesn't have to be modular but people tell me a good brand is essential.

Also I am also not 100% my mobo will take the 2 GPUs on my manual it says that it is Xfire certified but the thing is both the 4870 and 4890 are not listed in the GPUs on the manual, so im not sure if it will run it...if i do need to change it i will when i get the 2nd card later on just wondering if you guys would know if it would work or not.

Thanks a lot in advance for all your help guys its well appreciated
XoN