Combining two PCs

It could be that the northbridge doesn't like different speed RAM.
OK to verify, you have a molex/SATA power connector connected to the HDD right?

First off, try unplugging the HDD (though it probably isn't the problem, it is less cubersome to physically disable).
If there is still nothing on the display, then take out the 2 modules of RAM that you added in. At this point, if everything is fine, then take out the two modules of RAM left in it, and replace them with the two modules that you took out earlier. If it turns on, then try putting the other 2 RAM modules (the other T2893 you have) into it.

To me it sounds like either a RAM incompatibility (since the Radeon Xpress 200 does use shared memory), or, hopefully not, something got fried.

Oh, does your PC beep a lot when it turns on? If it does, then it's RAM failure (it is just called RAM failure, don't panic).

Well, the first time I attempted to turn it on, I didn't have a HDD in just cause I wanted to see if the BIOS would still come up ands ee if the RAM was recognized.

The PC doesn't beep at all when it turns on. It is weid that this happened with two seperate PCs, too. :/
I didn't touch the RAM on the 3406, either.