The ps3 version has some bug in it from what ive heard, so if possible you might want to get it for another platform.
If the PS3 is the only console you have and your PC can't run it well then definitely get it. But if you have a 360 or even better a good PC to run it on then get one of those versions.
The PS3 version doesn't have as good graphics as the others, but its still an amazing game.
My PC spec is in my sig... what do you think my chances are of running it at a reasonable rate?
I'd recommend waiting until they patch the PS3 version which is going to happen some time as they are adding trophies.
I guess the other consideration is the modding community. Oblivion on the PS3 had no modding or even updates... it's plausable that PS3 Fallout 3 will be equally ignored.