Installed new RAM: now my computer wont recognize my HD!?!?

Hello there everyone, thanks in advance for any help you might be able to provide! Here are the specs for my computer and my situation:

Windows XP
3Gig CPU
2 Gigs RAM (2 one-Gig sticks of DDR2 RAM)
GeForce 6800 Video card (I think it?s a 6800 ? Im not at home right now to double check)
80 Gig HD

And I think that?s all the important info? If you read my problem and need to know more to help diagnose it just ask!

I use my computer at home almost exclusively for gaming (Battlefield 2 mostly) and have had little or no problems at all for the last year and a half or so. However, a couple days ago I noticed that BF2 was starting to get all choppy. I had a good, fast, internet connection so I turned the graphics settings way down and tried to run the game on single-player mode and the problem persisted.

In addition to this my computer started randomly shutting down. I would be playing a game or installing a game or something and the darn thing would just shut off completely and without warning.

I was stumped. I couldn?t decide if the two problems were related or not. I asked a few friends in the tech industry and googled around a little bit and the best hypothesis I could come up with was that there was a problem with my RAM and I needed some more.

My 2 Gigs of RAM had always ran the game well in the past but I understand it to be a very memory-heavy game and figured an upgrade in RAM couldn?t hurt. I also hoped that if I got new/more RAM then the random shutdown problem would end as well.

So last night I go to Best Buy and get two sticks of DDR2 RAM that are 2 Gigs each. I go home, install the new RAM and fire up my computer. Everything is working great except that Windows is telling me that I now have 3.12 Gigs of RAM instead of the 4 that it should have. I ignored that for a moment and fired up BF2 to see if the new RAM helped my gameplay ? and it certainly did! The game ran much, much smoother and it seemed to have fixed the immediate problem.

However my stupid OCD kicked in and I wanted to know where my missing .88 Gigs of ram were! So I took one of the new RAM sticks out to see if one or the other might have been defective. I started the computer with only one stick in (slot 1 I believe on my motherboard) and when I checked XP it told me I had exactly 1.0 Gigs of RAM. This was interesting so I moved the same stick to the other slot (I thought maybe one of the RAM slots was dead ? and thus the motherboard would be the problem) and ALL HELL BROKE LOOSE!

Suddenly when I tried to start my computer it quit recognizing my Hard Drive completely! In fact when I turned my computer on it stopped before it even began booting and told me in DOS to ?insert a boot disk and press any key? I was shocked! So just for kicks I put in the Windows XP disk and turned on the machine. This time when it booted it asked me if a wanted to install XP and when I said yes the computer gave me a message to the effect of: ?Hey idiot there isn?t even a Hard Drive on this computer. Install one and try again.? So needless to say I am SUPER confused!

At that time I tried switching the RAM back to its original spot and tried every conceivable combination of RAM you can think of: both new sticks in and only one stick in either spot ? NOTHING HELPED. (The HD seems to be spinning -although no lights come on the front).

So to summarize: I have attempted to upgrade my RAM (successfully so it seemed at first) and then after taking one stick out and having no problems I switch the stick to the other slot and now my computer doesn?t recognize my HD! I?m not sure what it could be and would love to see if anyone else out there might be able to help me out?
Sorry for posting the Declaration of Independence here I didn?t mean for it to be so long but I wanted to be as clear as possible. I should be able to check the forums regularly today while I?m at work.

Again THANK YOU for any and all help!!