Dont worry about the motherboard, you provided enough information. Im guessing the DIMM that came with the PC is 64megs, and your PC has 8megs of shared video. Try the following:
1) start by taking 1 of the new DIMMs and putting in the PC, see if it boots, if it does and it reads ~120mb, then I would assume that DIMM is ok, take it out and set that one aside and try the other one with the same procedure. If the 2nd DIMM works, keep it in, and install the 1st DIMM you tested back into the PC. If that boots up properly and gives you a total memory size of ~248mb, you can install your original DIMM in the last bank and turn it on and see if it boots. If it doesnt boot with the 64meg DIMM in, and it booted before with the (2) 128's then keep the 64megs out and set it aside. I dont know what you use your computer for, but I doubt you will use up 312megs of RAM. If your PC has alot of dust in it, pick up a can of compressed air and give it a good cleaning, give special care to the DIMM banks, dust and other crap (cat hair) can get in there and cause the DIMM not to make a connection to the motherboard.
If you find that one of the DIMMs doesnt work (turn the PC on and nothing happens, or starts beeping)you probably have a bad DIMM. What you might have to end up doing is only using the (2) 128mb DIMMs. What kind of new memory did you purchase?? Kingston, Micron, PNY??