thats not the point, you can change quality settings on a pc, if it can do better than a PS3 now, it will always do better than a PS3, because games of the future turned down a bit will still look as good if not better than current games on PC which are better than PS3.
either way the 4850 will last you a while.
here is a screeny from COD5 on PS3:
http://image.com.com/gamespot/images/2008/233/944198_20080821_screen005.jpg
here is roughly the same part on PC:
http://image.com.com/gamespot/images/2008/191/944197_20080710_screen001.jpg
it's clear that the PC is much better, and that is an old graphics engine its using for the PC, its the same engine as COD4, so it'll run well over 70FPS on a 4850 at full settings.
on games on the PC there is games with much better graphics than COD5 though, and the screen shot of COD5 for PS3 above is pretty much the maximum graphically that the PS3 is capable of.