Possible Compatibility Issues When Upgrading 5+ Y/O System

[beerhelmet]

Hi,
The following are the specifications pertaining to my questions:
MOBO: Asus A8N-E
CPU: AMD Athlon 3700+
Memory: Old dusty stuff I can't read, but appears to not be the problem (4 sticks totaling 2GB DDR)
HDD and Optical Drive: Not connected, shouldn't be the problem either (500GB SATA and an IDE DVD drive respectively)
GPU (Old): GeForce 7900 GS
GPU (New): XFX 4890
PSU (Old): 480 W psu (Don't know the brand and appears to be dead when tested on another computer)
PSU (New): 700 W OCZ GameXStream
PSU (Substitution): 480 W Black Steel PSU
Case (Old): Antec Sonata II
Case (New): Antec 300 (Temporary case for purposes of testing power button)

Basically I want to upgrade my computer over the next 3-4 months.
My first attempt was the GPU, but that also required a new PSU, since my old one was not up to snuff.
After I attained these two parts I installed them in my old case without the old GPU and old PSU.
When I plugged in the PSU, the green light on the MOBO went on and I proceeded to press the power button on the case.
Nothing happened.
Well I figured I had installed something incorrectly so I removed the new PSU and new GPU and put my old PSU and old GPU back.
Not even the green light came on.
So from there I ended up doing a few things in an order that I don't currently remember so I'm just going to list them.

-Put old PSU into another computer and found that it was dead
-Tried removing an item at a time from the MOBO to see if it would turn on at some basic level and ended up with just MOBO CPU and PSU, still didn't work
-Installed computer in Old and New case for testing power button, cases appear to both function properly
-Borrowed a friend's spare PSU (the substitution) and the computer can turn on with MOBO, CPU, Memory and Old GPU (haven't tried it any further cause I was exhausted and it was 3 A.M.)
-Checked risers (standoffs), screws, etc. that may interfere on the MOBO and they appear to be properly installed
-Made sure 4-pin CPU power connector was installed from new PSU (labelled as CPU1) -> no help there


So, to get to the heart of the matter, my question are thus: Is there some sort of compatibility issue between my MOBO and the PSU, GPU or both?
Regardless of the answer to the previous question, what are my best options for upgrading my MOBO, CPU, and Memory that will be compatible with the new PSU and new GPU for a price range of around $500?
Will I have to reformat my HDD to use with any/all upgrades?

If there is more information that needs to be provided do not hesitate to ask and I will try to provide an adequate answer to the best of my ability.


Additional:
On the how to ask for new build advice they recommend to include the following information:

APPROXIMATE PURCHASE DATE: Between now and 3-4 months BUDGET RANGE: $500 After Rebates

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Gaming, watching movies, surfing the net

PARTS NOT REQUIRED: Keyboard, mouse, case, monitor (Samsung 2443BW), HDD (hopefully), DVD Drive.
I also have a new GPU and PSU listed above (XFX 4890, OCZ GameXStream 700), but am not sure about compatibility with these.

PREFERRED WEBSITE(S) FOR PARTS: Tigerdirect.ca, but can deal with most US websites and pricing COUNTRY OF ORIGIN: Canada

PARTS PREFERENCES: Anything that works!

OVERCLOCKING: No CROSSFIRE: I would like to in the future

MONITOR RESOLUTION: 1920x1200

ADDITIONAL COMMENTS: I don't want to spend much, or anything on cooling (hoping that built-in fans on case, fan that comes with CPU and GPU will be enough)

 

[False_Dmitry_II]

Well I would tell you that it's not a good idea to use a graphics card that powerful with a single core CPU. But it should still run. Did you try the new PSU in the other computer? Maybe it's dead. I'd say a 770 chipset gigabyte with USB3/SATA3 ~100 without ~80. (I'm talking newegg.com, don't know how close .ca is to that) With a athlon ii x3 for ~80 and 4 gigs of RAM ~100.

Then I'd leave your old stuff in the old case and sell it or something. Like I said, the graphics card and PSU should function just fine in either the old system or this new one, it would just be limited in the old one.