Hi diy
Your 3D view is pretty. Maybe I should do this sometime in Altium Designer.
The answer to your question is that is that it depends. In general, you probably only need the internal memory.
Since you are just starting, I would stop with the 21261 and move directly to either a 21479 or 21489. This gets you a much better part for less money. The SHARC code is going to look very similar.
You get substantially more internal memory with the 214xx which makes your question easier to answer. In at least 80% of all applications I have seen, the internal 5Mbits is enough. The exception tends to be the implementation of long delay lines. I would definitely avoid adding SRAM to a 2126x. If you are certain you need external memory, use a QFP176 version of the 21489 and use plain vanilla SDRAM. I commented on these tradeoffs recently in another post.
Both the 21489 & 21479 come in QFP packages, which allows you to hand solder your board. You will want a large heat slug hole if you are not going to use automation. You must solder the heat slug. If you use the 100 pin QFP, either the 21479 & 21489 could be used on the same board. This would allow you to trade off of power consumption versus speed. You would need to make sure that the core supply is sufficient for the 21489 in this case.
Al Clark
www.danvillesignal.com