Because you said that you plugged the computer in to two good monitors it has made me a bit more curious. Otherwise I'd question the memory. Now, I don't know a great deal about PC hardware, so please, if I make a mistake then I'll accept that. Are the memory modules both of the same type (e.g DDR, DDR2 etc) and do they operate at the same frequency? If not, I would suggest removing the extra RAM and seeing straightaway if that fixes the problem or not.
Or, you can see if coincidentally your video card has busted at the same time.
If you follow the wire from the monitor to where you plug it in to the PC, and take it out, check for any pins that have come out. Also, when you took the computer apart to add in the memory, did you knock or bump the video card?
Also make sure you do not exceed the machine's allocated RAM limit.
Best of Luck
I have a Gateway E4100 desktop with XP pro SP2 installed. I added more memory to it and when I started up the computer it started beeping. Everything seems to power up(cd-rom,cd-rw,hard drives,fans) but it's not sending a signal to the monitor. I know the monitor is good and I plugged the computer into another known good monitor also, still nothing. I removed the new memory and restarted and still the same. Anyone have any ideas?