The 3850s suck up a bit of juice. Even though your PSU claims it's a 500W unit, it may simply be a cheap unit that doesn't supply enough power. How many amps does it have on it's 12v rail(s)? Also, those cards are very long for AGP slots. You have to make sure it's properly seated in the AGP slot. On my older system, even if my 1950GT AGP looks like it's seated properly, it could be slightly off making me reseat it and test it again. The 3850s are longer and heavier which make that even more likely.