Dimension 4550 does not BOOT after checking memory type

I was trying to upgrade the memory on my Dimension 4550 P4 2.8Ghz 433 FSB. I simply opened the case to check the type of memory and then closed it. I go to startup the pc and notice nothing is coming up ( no POST) on the screen. When i hit the startup button the various cd/dvd drive lights flash on and i hear the fan running, but the hard drive light does not flash. It just stays on constantly as if it is NOT reading the hard drive. Keep in mind I DID NOT touch or move anything else initially other opeing the case to check the memory. The DELL error codes on the back indicate a problem with the memory modules, but i tried that same memory on a DIFFERENT pc and it was recognized on the other pc. I've checked the IDE connections, power cable connections, etc. in an attempt to fix the problem. FYI when i tried good memory from other PCs my mothboard starting making noises. I suspect the mobo is shot! I don't know why considering I never EVEN touched it in the beginning. If it is the mobo can i upgrade to a similar DELL mobo? I'm thinking it might be cheaper to sell the working parts of this one and use the money to buy a new one altogether.  Please help......THANK YOU in advance!