Memory size has little to do with performance, often you should get the card with less memory but better in other specs, for instance you should get a 128 mb radeon 9800 vs a 256 mb 9200. (which are both old cards)
Which versions are u talking about, because like I said above if you see a x850 with 256 mb, your not going to get even close performance with a 9200 256 mb. So no you cant save money like that as there are drastic differences sometimes.