That's harder to diagnose. i don't think it's overheat. I recalled mine have incidents of random crashes prior to hard froze on start up. Crashes are usually sign of hardware failure related to problematic ram on its way out. If you have time, do a dos command memtest to stress the Rams, run it overnight and you should able to pin point if Ram was the culprit. You can also check the system log after crash if it tells what was the cause.

i was under the impression that u only experience random crashes now and u can boot it up. No, you can't do a memtest if you can't boot it up

Double check both the memory raise cards and make sure all sticks are both set and lock it. Then do a SMC and PRAM reset (search for the instruction if u don't already know)

If it still doesn't boot, then it's a good time using the previous Ram stick one by one test and see which one has the problem. If it does, run it bit longer till u figure out which one has issue or process to memtest. There are two raise cards, try swamp and change stick on different card too. It could also Ram problem only when number of sticks are used together so your tests should cover that as well.

I can't recall if both needed to be inserted. it was long time ago when i owned it. I believe top riser must be used if your system only have a pair. 

I do think Ram was your problem. Mine started with same problem, occasionally crashed then froze at start up. When it starts to freeze more often, that's where i had it. See with Mac, you don't always restart your machine, sometimes for month! so you don't notice start up problem until it crashes and can't start. 

Macpro 2008 Ram i think runs in dual channel mode. So you must used a pair on same size next to each other and since you have diff size in pair. you have to be careful not to mix match. see the spec here
https://manuals.info.apple.com/MANUALS/0/MA...oryDIMM_DIY.pdf

This is the combo i would try for testing

Test 1
Top Riser : 2x2gb
bottom : none
 
Test 2
Top Riser : 2x1gb
bottom : none

Test
Top Riser : 2x2gb, 2x1gb
bottom : none
 
Test
Top Riser : 2x1gb, 2x2gb
bottom : none
 

Test
Top Riser : 2x1gb
bottom : 2x2gb
 

Test
Top Riser : 2x2gb
bottom : 2x1gb

at this point you still can't figure out the start up issues, then try runs memtest on test1 and test 2, if still nothing then go back to combo that gives you problem and runs a memtest

No, you can't

Have you try to reset smc and pram?

Just to make sure you done right. SMC reset required unplug power and wait for at least 15 seconds. I did that every time i got the riser issues on restart. That was the general SMC reset for Mac.

If you don't have the services manual, there are more lengthy step of reseting on the Macpro (Real Time Clock RTC Reset) and RED light trouble shooting guideline so you can pin point the issues
http://tesselator.gpmod.com/_Text/MacPro_Service_Manual.pdf

Hi Bro. I really thank you for the guide and the super useful troubleshooting link!
According to the Diagnostic LEDs chart, when I pressed on Power On Button, immediate it shuts down and I monitoring the LEDs... I noticed that the LED 4 & 5 are Flashed Once (every time when I powered on then it shuts down). As the Chart explain, my Mac Pro might facing CPU Over Temperature (Overheating?) And the Troubleshoot as :

Verify proper heat sink installation.
Verify all thermal sensors are properly connected.
Verify all fans are operating properly, especially the front intake fan.

In fact, I have no idea how to check...haha.

Hope you may give me some ideas? Thanks!

** In fact, I just leave it for whole day, never even boot up as it immediately shuts down every time when i was powered up. How could the CPU Over Temp ?

First make sure you properly reset the SMC otherwise the error will always be there.
Blink once probably not the issue. It should stays solid

There's a SMC Reset switch on the board if you have not tried it. Page 113

Try remove GPU and see you can boot from it. I think the machine can boot if GPU is removed, just not when it's bad. May be see you can hear the chime or see the network shares if it does boots.

If not, at this point, i would remove all attached hardwares and try do a simple test one by one

any usb devices, remove it
remove hdd
remove cd drive, cable should do

That leaves possibility of power, logic board and riser cards. If your logic board is bad, it would be way too expensive unless you can find a used one. U have to find local apple service stores who has spare parts to do further diagnose. I have a macbook pro one i spilled water on it and the logic board gone bad. The cost to replace one is as costly as a new macbook pro. Good thing it was under warranty and i dried the darn thing before bring it in.

Good luck, man.

Let's put thing in perspective here.

There's not much value in 2008 Mac Pro. I sold mine for $450 USD with 16G Ram, SSD, upgraded GPU.  If i had have a bad logic board, ordering a new one would cost me probably 3-4 times more it values. I would rather not fixing it if i can't find a used part.

I would call in Apple center and find out how they charge on diagnosing and repair cost. Then I would bring in local computer stores who has spared parts and can pin point the issues. If they manage to find out the problem and bad hardware, then you will decide what's more cost effective. Save the money, buy a faster new Mac or bite the bullet.

But don't give up yet. I went though a lot when the GPU went out, i was ready to throw in the towel on logicboard or riser card and turn out it was the GPU. I was lucky to find a guy would let me test on his spare video card before buying from him.