I'm lost

I just got a new system and I am having problems with my video card MSI RADEON 9550 when getting into the heat of battle on the newly released Game {TRIBES VENGEANCE} i AM GETTING BEEPS the kind of beep you get when your VC is not seated( it is seated) I was told to try to tweak it with "RIVATUNER". I DON'T HAVE A CLUE WHERE TO START???? WOULD SOMEONE GIVE ME DIRECTIONS, PLEASE
the rest of the system

cpu: intell celeron 2.7/w extreme cpu cooling
ram: 1 gig of pc2700 ddr ram
hd: 80 gig
power sup. 420 watt
mobo: asrock p4s55fx+
cdrw/dvd/ 3 1/2 floppy
3 xtra cooling fans
operating system: windows xp pro
I realize it is nothing compared to what most of you have but I had a p2 333 mhz 256 mb, so this is a big step up for me
I would be very grateful for any help

your cpu may be heating up to the point where the heat sensors on the MB are telling the bios that you are getting close to the danger point. look into your bios on startup, del or F2 most of the time, and goto the power section. In there you should see something for monitor alerts or such, look in there and it will tell you how hot the system is right now, and see where the alerts are set at. it may be that it wasnt enough of an upgrade.... was it doing this before?, oh almost forgot I know that the PS is new and all, but it may be that you have a borderline bad unit, and power flucuations are causeing the error.