How do u think of my spec and I want to upgrade it (Needed Advise)??

To be honest, I wouldn't mess around too much. It's not that bad of a computer besides the dual core, but I wouldn't spend $200-300 on a CPU for a dead socket. I certainly wouldn't spend anything on a new case.

What resolution are you using? Is this a gaming computer?

About the only thing I would change is the GPU. Judging by your suggestions, you're looking to spend about $600 (might want to give us a budget). I would get an HD 5870 for $400. This is one of the best cards out there, but is completely overkill for any resolution under 1900x. If your resolution is lower than that, the 5850 or 5770 would be best. The 5870 and 5850 will both be good in a couple of years, and the 5770 is great in Crossfire.

On the HDDs, I would get just one Samsung Spinpoint F3 1 TB for right now. I assume that will hold you for a year or two. The reason I wouldn't buy a massive amount of space right now is that you will probably need to rebuild entirely in a couple of years, and by then, faster, cheaper drives are sure to be out.

Both of these upgrades will improve gaming performance right now, and will be good to carry over to another build. Given the LGA 775 socket's death at the hands of the i3 CPUs, you're most likely going to need to completely rebuild in a year or two. Start saving up...

EDIT: Forgot to add the heatsink. I'd get a Coolermaster Hyper 212. It's excellent, pretty cheap, and works with every socket in use today (LGA 775, 1156, 1366, AM2/AM2+/AM3). It's another part that will easily carry on to a future build.

yes , this is a gaming computer..
My resolution not so high just 1440X900..
I planning to buy a Seagate Barracuda 7200.12 1TB or Western Digital Caviar Black 1TB
And I planning to change my GPU to HD 5770..
And if I want to change my Mobo to LGA1156 socket.. I planning to get a MSI P55-GD80 and a Core i5 750...
Any mobo u recommend ?
I try to get V10 or V8 for my CPU...

For the HDD, get a Samsung Spinpoint F3. It's the best drive out right now. The Seagate is a suitable replacement if it's out of stock.

The 5770 would be a great choice.

I like to recommend the Asus P7P55D-E Pro. I still think that you should hold off on upgrading the CPU for a bit. The main reason is that you should still be getting good performance from that dual core, and the later Intel sockets are going to be replaced when the next CPU comes out.

All I would replace is the video card -- probably with a 58xx series, which you'll be able to carry over into your next system. I wouldn't do the motherboard/CPU for another year or so. You'll see a lot more/better or at least cheaper options in the i core range. The 5770 also won't be quite so good to carry over then.