Motherboard

If you compare a computer to a human body, the motherboard is like the spinal cord. It's what you plug the CPU, graphics, memory, and other stuff into. It's also what gives you most of your ports, such as USB and sound. While it's hard to quantify its impact on your performance other than by measuring bus speeds and such, It really does have a huge impact on the stability of your computer. My advice is to go to newegg and read the reviews on them, or get a nerd friend to help you pick one. Are you building your own computer?

yea,how will I know what the motherboard is in the system above.I'm buying it from dell

Well, if you're buying a Dell computer, it will have a Dell motherboard -- it's what's called a proprietary component, basically meaning that the upgrade possibilities will be limited but what's much worse, if your mobo fries, you'll have to replace it with a Dell mobo... and that's where they get you.
Proprietary parts are usually more expensive than the other ones - the good ones.

Another significant aspect of motherboards is that it's the component that uniquely identifies the computer during Microsoft operating system (OS) registration/activation. Replace the motherboard and you have to call mother Microsoft and explain it's a hardware upgrade of your existing OS licence. She only lets you do that once. I'm migrating to windows 7 in October and will upgrade to a PCI-E v2 capable motherboard just before.

With respect to motherboards, it's all in the input/output (I/O) chips. Your "specked" Radeon™ 4870 and Dual Channel DDR2 SDRAM denotes a current generation I/O chipset. As always there are break throughs on the horizon.

Ok,thanks I know what it is now but do you think this pc will run fsx Ultra high settings with high framerates

It's a pretty wicked system i'll give ya that... Though I don't know much about ATI cards so you might want to double check that it's a good one.

Yes i imagine that this computer will run FSX with pretty high settings because it has a processor with lots of GHz and a good graphics card!

Hope you enjoy the new computer!

just got my new pc.Runs fsx extremely well.really smooth.thanks for all the advice