|"....there is no video..."|
No video at all, or you have video for the inital part of the boot, then no video after Windows is supposed to load.
The latter is easily fixed !
ATX mboards are always powered in some places, even when they are not running, as long as their ATX power supply is connected to it, the PS is switched on if it has a switch, and the PS has live AC power to it.
Did you unplug the PS/case, or otherwise switch off the AC power to it, at ALL TIMES when you were fiddling with connections or components included cards and the ram inside the case? If you didn't, things can easily be damaged !
Simply plugging in or unplugging the video card while the PS has live AC can fry the card, and/or the PCI-E X16 slot circuits of the slot it's in, or the PS !
No video at all.....
If the video card has never produced video, assuming it's not damaged, and the slot circuits for the slot it is in is not damaged...
(you could test the card in another working system) .....
Some recent and fairly recent mboards do NOT automatically disable the onboard video when you install a PCI-E X16 card in a mboard slot, AND the video card will NOT display, by default. If you have that situation, you will find that the onboard video is still working when you have installed the PCI-E X 16 card, when you connect a monitor to the onboard video port. If you have that situation, when you have connected a monitor to the onboard video port, you need to go into the bios Setup and set the Primary Video or Intialize Video First or similar setting to PCI-E or similar, save settings, and the video will then work when you connect the monitor to the card in a slot.
"....gives me no system beeps..."
You must have a case speaker connected to certain pins on the mboard, or have a built in sound device on the mboard. which is rare these days, in order for you to be able to hear mboard beeps. See your mboard manual if you not sure whether that's connected.
You should hear one mboard beep when the POST completes successfully while booting, if nothing's wrong.
"I have already replaced the power supply, motherboad, and memory."
A defective power supply can cause no beeps.They often partially work, fans and hard drives may spin, leds may come on, yet you may get no video and the mboard will not boot all the way.
"I have tested the power supply"
The PS can easily be tested by connecting it to a known working computer, but other tests such as the paper clip trick or one of those led power supply testers do NOT necessarily indicate there's nothing wrong with the PS.
If the PS was new, it's extremely unlikely there's anything wrong with it, unless you have done someting that damaged it, or it has been exposed to a destructive external event such as a power surge or spike, a power failure, a short, or a static electricity discharge, etc.
It's extremely unlikely there's anything wrong with the original mboard, or your cpu, unless you did something yourself that damaged it.
The fact that the onboard video worked probably indicates there's nothing wrong with it, unless you have damaged it - you may have damged the video card, or the PCI-E X 16 slot circuits.
It's extremely rare for ram that worked properly previously to suddenly go "bad" , and even more unlikely for more than one module to go "bad" at the same time.
There's probably nothing wrong with the ram that worked fine previously! Almost always, if you have a ram problem, it's either because the ram is not 100% compatible with you using it in your mboard, or the ram has a poor connection the the ram slot(s) !
Trying ram that is different from what worked fine previously may cause MORE problems.
Ram that works in another mboard , or any ram you buy or have lying around, may not work properly, or sometimes, not at all - even if it physically fits and is the right overall type (e.g. SDram, DDR, DDR2, etc.; PCxxxx, xxx mhz) for your mboard. In the worst cases of incompatibilty your mboard WILL NOT BOOT all the way with it installed, and the mboard may not even beep - the ram has to be compatible with the mboard's main chipset, or in the case of recent mboards, compatible with the memory controller built into the cpu.
If you still have the ram that was installed when the system worked fine, try installing just that ram.
See response 5 in this for some info about ram compatibilty, and some places where you can find out what will work in your mboard for sure:
Correction to that:
Once you know which module ID strings work in your mboard, you can get them from anywhere you like that has ram with those ID strings.
If you have brand name ram, it is usually easy to look up whether it's ID string is in a list of compatible modules found by using your mboard or brand name system model number.
If the ram is generic, that may be difficult or impossible.
It is easy to test for incompatible ram that has caused your mboard to fail to boot, or boot fully, or ram connection problems that cause it to not boot properly .
Make sure you have a speaker or speakers or the equivalent connected to the mboard so you can hear mboard beeps (see your mboard manual if you need to).
Remove the AC power to the case/power supply.
Remove all the ram.
Restore AC power.
Try to boot.
If nothing else is wrong, you will get no video but you will hear a pattern of beeps that indicate no ram is installed, or a ram problem.
E.g. for an Award bios or a bios based on one, that's often a beep of about a half second, silence for a half second, a beep of about a half second, silence for a half second, continuously.
A common thing that can happen with ram, even ram that worked fine previously, is the ram has, or has developed, a poor connection in it's slot(s).
This usually happens a long time after the ram was installed, but it can happen with new ram, or after moving the computer case from one place to another, and I've had even new modules that needed to have their contacts cleaned.
See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated: