There are some major differences in components and size that make these items hard to do a direct comparison, but value in different. The new iMacs will have DDR3 RAM while that Dell has DDR2, but most importantly is the value difference between a tower and an AIO. If we are jsut comparing CPUs then the Dell would win, but that is very narrow view in which to make a purchasing decision.
DDR3 provides no benefit for Core 2 Quad systems. In fact 1066 DDR3 memory performs worse compared to 1066 DDR2 because of the increased latency in the memory chip. Core 2 processors are bandwidth starved due to the memory architecture and using the ancient FSB way of accessing the memory. No matter how much you increase the memory speed, it will not the overcome the bottleneck created by FSB. All of benchmarks done on websites prove that DDR3 offers no benefit on Core 2 platform. Plus any DDR3 speed above 1066 will require extra voltage and iMac cannot afford that
Core i7 benefits from DDR3. It leaves the FSB concept behind and instead uses on chip memory controller which significantly increases the memory bandwidth and reduces memory latency.In fact, according to benchmarks, a single channel DDR3 on i7 is able to match and exceed dual channel configuration on Core 2 platforms. It also uses DDR3 in triple channel configuration which also helps to increase performance even more. Also truth to be told, the triple channel offers little improvement in real world situations such as games or programs. This is why (besides the architectural benefits) the lower end i7 is able to outperform the Core 2 Quad Extreme Editions (at 4X the price)
Apple's iMac is very much behind the competition in terms of spec and power. Core 2 Duo might be enough for email, iPhoto, Safari and Photobooth, but a computer at that price MUST have a quad core system considering what the competition offers. In PC world, quad is the new standard. Apple has a long history of innovation and providing high performance computers, they should go quad on all desktops