Why it is that they would stick with a 256 bit memory bus (aside from the fact that clock for clock its really the same speed as a 512 bit bus of slower memory?) Is it just because the rest of the card is a bottle neck? I don't think I can recall another card, that when all other things were equal, a faster bit bus didn't result in a sizable increase in processing power? It was obviously implemented in the previous generation of cards, so why not stick with it, use the GDDR5 and then end up with a card thats even faster?

Can anyone explain to me why they would do this (or not do this, depending on how you look at it?)

According to the article at anandtech the following are still unknown: Clock Speeds, Power Usage, Pricing, Performance. Pretty much the breakers are unknown.

This monster is already 550 mm^2, I don't think the couple million transistors needed to do a 512bit bus would be noticed, nor would the cost of the pins to connect to the outside. The more likely explanation is that they aren't memory starved and that trying to route the extra high precision lanes on the board was either too hard or was going to require more layers in the PCB which would add significant cost.

No, the question is:

Is the price / performance difference worth the investment in the pricier card, or does opting for the cheaper option allow me to buy a case which will fit the card for a net saving?

If GF100 price > 5870 + New case, you have an easy decision to make.

With AMD's 5000 series is now coming down price, by the time NVIDIA gets it's 100 series shipping, it won't be *that* much faster since it's a similar generational leap, and similar process size, but it will be high priced until a significant ammount of stock hits channels... oh and to sting the early adopter fanboys.

Just like the sucess of AMD 4800 cards, many people will go for the significantly better bang for buck in the 5800 line. It looks like AMD is in a good position.