In essence, the Nvidia GTX 560 is very similar to the 460. While the 460 used a crippled version of the GF104 GPU, the 560 uses a cut-down version of the newer GF114. The cards are functionally identical -- the GTX 560 is just newer and faster. There's the same number of stream processors, ROPs and SMs, and the same size L2 cache and memory bus. In fact, the only real advantage the 560 has over its predecessor is clock speeds: the core, at 810MHz, is 20% faster than the 460; the memory, at 1GHz, is 10% faster.
Interestingly, the GTX 560 is the first mid-range product from Nvidia that has launched without a reference card. Nvidia and AMD usually ship a reference product with stock speeds, so that partners like Asus and MSI can offer factory overclocked cards -- but for the launch of the GTX 560, you can only buy factory overclocked cards. This means that, while Nvidia lists what the stock clock speed of a GTX 560 card should be -- 810MHz core and 1001MHz memory -- the cheapest card you can buy has a core clocked to 850MHz, and memory at 1026MHz. As a result, only cards with the smallest factory overclocks will be around $200 -- the rest will cost more, and begin to creep towards GTX 560 Ti territory.
Performance-wise, the GTX 560 is excellent -- it's a clear upgrade from the GTX 460. The lack of a reference card means that your mileage will vary, though. The top-end GTX 560s are comparable to the GTX 560 Ti -- but the mid-end GTX 560s are tantalizingly close to the cheaper Radeon HD 6870 on some games. As always, it's well worth checking the benchmarks for your favorite games before investing in a GTX 560, or indeed any new graphics card.
Finally, it's worth noting that the GTX 560 is also slightly moreenergy efficient than the 460: even with a 20% higher core clock speed, Nvidia puts the TDP of a "stock" GTX 560 at 150W, a full 10W less than the 460. If you're realy concerned about power usage, though, the Radeon HD 6950 still draws a whole 20 watts less than the GTX 560.