The best processor? Well it depends on what u need it for. But for overall (gaming, multitasking...) id suggest Amd X2.
Ram is different from CPU.
Yes, CPU is the same as processor
No, ram is the memory in ur computer. So it's different from the CPU.
2x7800GTX, for this configuration u need to have a motherboard that supports SLI.
2Gb of ram is very good. To power it? Well if I understand that correcly, u will need PSU (power supply) that gives power to all of the parts in ur computer.
No, harddrives are connected to the mobo with cables, either IDE cables or SATA cables, depending on which one the harddrive uses. All of the new motherboards should be compatible with both IDE and SATA.
I don't know much about sound cards but if u just listen to music and play games u don't have to have a very good card.
Well I gues u can't say which is better AT or ATX. But the more common one is ATX so I'd suggest u get that.
Yes u do need a cooler. And yes they come with one. U can also buy better ones but if u are not going to overclock the stock cooling is going to be fine.
Some cases do have a PSU. Some don't. But if u buy a case with a PSU make sure the PSU is good, something like 450W or 500W for u since u are building a powerfull system.
Fans are optional. They do keep the parts cooler, and the cooler the better performance. And usually cases have places to put them to.
Hope this helps.