I think it is pretty amazing that NVIDIA has managed to put two GT200 GPUs and supporting components onto a single PCB. While this is certainly impressive from an engineering perspective, that change alone doesn't benefit the end user much. Due to the new cooler used, the card is actually quieter than the dual PCB design but from a rendering performance perspective there are no improvements. NVIDIA has chosen to not increase the clocks the slightest bit.
For the AIBs there is a fundamental improvement with the new GTX 295, it is much cheaper to manufacture. The major cost savings come from one less circuit board, cheaper voltage regulation circuitry and less complex manufacturing process. This will allow substantial price reduction on the GTX 295 in the future, which will benefit you as a customer. At this time there is not much to see of these price cuts yet. The single PCB GTX 295 costs as much as the dual PCB version right now.
So if you already have a GTX 295 there is no real reason to upgrade to the single PCB version. If you are in the market for a new card, the single PCB GTX 295 is the better overall choice, except if you plan on using software based voltage control on your card. Then the dual PCB GTX 295 should be your weapon of choice.