So if a graphics card barely manages less than 30 FPS, then the game is not very playable, we want to avoid that at all cost.

With 30 FPS up-to roughly 40 FPS you'll be very able to play the game with perhaps a tiny stutter at certain graphically intensive parts. Overall a very enjoyable experience. Match the best possible resolution to this result and you'll have the best possible rendering quality versus resolution, hey you want both of them to be as high as possible.

When a graphics card is doing 60 FPS on average or higher then you can rest assured that the game will likely play extremely smoothly at every point in the game, turn on every possible in-game IQ setting.

Over 100 FPS? You either have a MONSTER graphics card or a very old game.

with rm7900 can get a high end pc with custom water cooling setup

Your posts are horribly hard to read but I'll do my best trying to understand them...

1) New hard disk. A WD Caviar black or faster would be a good fit. Your IDE drive is probably horribly fragmented due to the lack of space.
2) Are you using the latest nvidia drivers? Or whatever was in the asus cd?
3) If SC2 made near full use of 6GB of RAM, you'd hear nonstop whining from players over it being a memory hog. Most gaming rigs barely use 4GB at most.
4) The GTX 480 is known to be both very powerful AND very hot.
5) SC2 is designed for dual core CPUs and thus wouldn't use the i7 975 to its fullest.
6) The Rampage 2 extreme supports SLI too, but I don't think you should add another GTX 480, the power supply will be really strained with that inside.

IMO, that's 7900 bucks poorly spent, you should've gotten a GTX 480 with a better cooler (hell, you could even buy another card for SLI) rather than spend on the i7 extreme that you don't even know how to make use of to its fullest.

My suggestion:
-Sell the i7 975 and replace it with an i7 930. You'd probably pocket almost RM2000 for that.
-Use the extra money to get a quality 1080p monitor (such as those Dell Ultrasharps) and a real hard drive.
-No need to upgrade your graphics performance, it's not like you can see 90fps on a 60fps monitor anyway.

Together With My Old Devices (I know is abit funny but I really run our of money.)
*IDE 40GB Hardisk
*And Those Old Devices(mouse, keyboard, monitor & etc)

*I Personally Don't Think The IDE Hardisk Affect Much To The Performance (Yes, but not much).

Change your HDD to a newer one like a Samsung F3 Spinpoint 1TB or Western Digital Caviar Black 1TB. Your HDD transfer speed is bottlenecking your com.

U r like running a BMW machine with a kancil engine...  just change your hdd bro... u will see a major difference