I can't picture a lot of people using registered memory in their game machines. A fairly recent sub-$200 motherboard that accepts registered memory is more rare than one of those $99 Best Buy GF3 cards. 🙂 Registered memory is used on motherboards that have quite a few DIMM slots - for PC133 this would mean more than 4 DIMM slots, for PC2100 DDR it would mean more than 3 slots. It has no other advantage beyond the fact that you can put a lot more DIMMs on a board that you otherwise could.
I'm the guy who's entry on ECC is used in the FAQ and I'm a pretty strong advocate for ECC, but even I don't advise gamers putting it in their machines. ECC is all about data integrity. If your the data on your machine is important enough to you that you are often doing backups, or that this data would really mess your life up if it disappeared, or if your life would be messed up by your machine being unavailable for a day or two, then you need ECC. If it would be a minor irritation, then you don't need ECC. ECC is not much more expensive (usually less than 10%), it barely affects performance (less than a 3% performance penalty in most cases), and if it saves your data just once during the life of your computer, then it more than compensates for itself. But if your data is not that big a deal, then it's a little harder to judge.
As far as 256MB vs. 512MB, only you can tell if 256MB is too little. If you are disk swapping often enough that you notice it, then upgrade to 512MB, if not then save your money.