If you have been changing out parts with power to the PSU then you may have killed the chipset. (Parts of the chipset are powered whenever the PSU has power. Replacing parts will cause voltage spikes in the chipset.)
Most 8x video cards are actually 8x/4x even though it isn't mentioned in the advertising.
The important difference isn't the AGP speed it's the signaling voltage.
(The voltage the data travels on.)
A 4x only board only supports 1.5v signaling.
An 8x/4x card supports 1.5v signaling as well as 0.8v signaling.
Try removing the add-in video and use onboard (for now) and reset the CMOS.
That should return it to defaults which will re-enable onboard video if the previous configuration w/add-in video turned it off.
Only slight possibility~~~
Some Intel D845xxx boards suffered from faulty Nichicon capacitors.
Your board is not on the list but if you have Nichicons with an HN or HM [may be followed by (M) which is the tolerance and not part of the model/series name] marked on them then your board could have the same problem.
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