vid card issues

[N0de]

recently I picked up an e8400 and all the supporting stuff because the pc came with a pci-e slot and was a cheap deal. I have an antec 500watt psu and a x1900xtx. The problem is that when I push the card or even when i tab out of a game after a bit the screen flickers. It does it sometimes in games. For example in UT3 I can't run over 1024x768 with gfx settings on medium without horrid shutters. Like FPS stays the same but the gfx flicker a lot. Im wondering if the PSU isn't enough to run the system under load or the gfx card is going out. Also from time to time i hear the fan kick on high then it beeps at me. Im at a loss but if it is going bad is there a comparable card from nvidia in the 100-150ish price range?

thanks.