The wikipedia entry for the iPhone has information about the iPhone's hardware components listed as below. Take note that these specifications came from other websites based on their skills in disassembling the device.
Processor: 620 MHz ARM 1176,underclocked to 412 MHz
RAM (Memory): 128MB DRAM
Storage: 8GB and 16GB
If only the iPhone's screen is large enough given the specs above, it can definitely run Mac OS X 10.4 or Windows XP with additional software tweaking. But you don't need to have that as the iPhone OS alone is terrific.