I have two questions.
My first question is, will ATI 4870 1Gb work with its full potential with my AMD phenom 9550 CPU?
I had problems with geforce 8500 1Gb (this card sux). My computer would freeze with that card. I checked everything, the card, OS, and my other hardware. And my computer would freeze only with that card, but it didn't with my friend's card, radeon 2600 256Mb. I guess geforce card didn't work properly because I had AMD hardware. And the question is, will my computer freeze with ATI 4870 1Gb. (I know, that's a noob question).
the 4870 does like a lot of cpu power, so the CPU may be a bottleneck for the GPU.
for the CPU speeds (unless u are willing to O\C) go for the 4850 as this does not require a powerfull CPU to gain the full potential, also this card with the latest drivers are catching up with the 4870.
as for the freezing,this could be an issue with drivers, a problem with the hardware or a problem with windows.
I recomend a full re-install of windows after the Graphics card replacement just to be sure.
for the most part it will be better than the 512mb version
although this will only make a difference realy if accompanied by a 24" or above monitor.
any thing bellow 22" monitors dont require 1gb of vga memory, though this may change in the near future.
as long as the price is right, and u dont plan on replacing the GPU often (otherwise a waste of the extra cash in conjuntion with the time release of the 1gb requirement of games as a standard), then 1gb hd4850 will be perfect.
also 2 in crossfire is spectacular, especially as most game from now on will cater for dual graphics as standard.
no your computer wont freeze it will work perfectly and your cpu isn't nearly as much of a bottleneck as these enthausiasts try to claim. even on a athlon x2 you would get the benefit out of the radeon 4870. i dont think you will even notice anything running intensive games sure with a faster corei7 or core2quad cpu a you might get a few frames more but this is an overkill costwise and not even worth considering since you already have a well build system Phenom 9550 isn't a slow cpu at all it is way faster then athlon x2. i recommend to get the card and carryon with you cpu and might upgrade it somehwere late 2009 or 2010 once the lastest phenom models are launched before they step up to another Socket this is the cheapest way to and will squeeze out as much as possible out of your current system.