A problem with a Dell Dimension 4500S...?

I have a Dimension 4500S. The other night I got the dreaded blue screen. No hardware or software changes hand been made in a couple of months - I was just on the internet when it happened. My husband thought it needed a reboot, so I powered down, turned it back on, and it won't go further than the Dell screen. We tried again, and it won't even go that far. I've read on forums that the video card is gone (the monitor - which came with it - just stays asleep when it's turned on). The diagnostics lights showed a memory error. I took both sticks out - tried both separately. After trying both, I have a new light - GGGY - which, via Dell, means another error. I have NO clue what could be wrong. Could it be the battery? Could it be the motherboard? Hard drive? Other issue? Any help would be greatly appreciated! :)