"All over" was wrong then. The size of a videocard's heatsink has nothing to do with it's performance. A single-slot 7950GT wipes the floor with a Dual-Slot X1800XT, for example.
I don't see any reason for a 256MB card to be worse than a 128MB card, but I also can't see why a 256MB card should be any better than a 128MB card on the FX5200 that you're going to buy. More memory is only needed when you run at higher resolutions, and you won't be playing at any resolution even close to what's needed to utilize 256MB. I daresay 64MB would be enough for a FX5200.