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Did you unplug the computer or otherwise switch off the AC to the computer at all times while you were fiddling around with components and connections inside the case?
Yes, you could, especially regarding re-seating the ram, but...
I searched using your model number - it's still being sold new recently, so it can't be very old.
"A cmos battery that is too weak or dead will NOT cause the computer to not boot!"
Coin shaped cmos batteries typically last about 5 years. It's extremely unlikely it would be too weak or dead. It isn't old enough yet.
Power supplies sometimes damage things other than themselves when they fail - often the mboard. It can happen with any power supply, but the cheaper the power supply, the worse the quality, the more likely that is to happen.Most computer related pieces are better quality now than they were in the past, but there are more bad el-cheapo power supplies now than ever before!
What brand, model, and wattage capacity is the original power supply?
The new power supply?
Your mboard has onboard video, but do you have a PCI-E video card installed?
Your power supply must have at least the minumum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD!)
You can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements.
If you want to cover any possible video card, a decent quality minimum 600 ot 650 watt power supply will handle any current high end video card, or even a X2 card (two video chipsets on one card) or two cards in two slots.
Don't buy an el-cheapo PS.
See response 3 in this:
If the mboard HAS been damaged, it's very likely nothing else has been damaged, so if you replace the mboard with the same model, or with a newer model that can definately be used with your same cpu and ram, there's a very good chance your system will work fine with all your other pieces.
If you want to rule out whether the ram has been damaged, which is extremely unlikely unless you installed it BACKWARDS in the ram slot when you were fiddling with the ram, which can also fry the mboard as well (you would have noticed a definite stink immediately after you attempted to boot the computer, some of the ram module contacts are damaged, the ram slot it was in is damaged - if you clean the melted plastic out of the damaged ram slot the mboard MIGHT boot after that)
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.