PLEASE HELP!!! New build -lock-ups, ram timings

I just built a system with these specs:

Core 2 Duo E6300

Corsair 2 gigs DDR2 800 ram

Asus 8800 GTS

My problem is kind of odd. The system will run fine for hours... cooling is fine. Then randomly it may reboot itself while playing this 1 game that I have for my GF (Rollercoaster Tycoon 3). At first I thought it was the game and it needed to be patched (original version, not patched). However, I shut down the computer as regular at the end of the day without problems. But then when I start it up the next day, the computer will go to the Windows log-in and lock-up. Other times, I get to log-in, it starts to load Windows start-up stuff (which isn't much since I don't have barely anything installed) and then it hard locks soon after before I get a chance to run software.

I went into my bios to adjust some things... I thought maybe it was a ram issue or a bios setting of some kind... so I went and manually adjusted the timings. Now I get 3 beeps and it won't boot at all.

So my questions are 1) What can I do to reset those timings if I can't get it to boot and 2) know of a remedy for my lock up issue?

Thanks in advance