Be careful -- the 6600/6800 are great in Doom, but the lead over ATI is much smaller in other games.
Your rig lists an A64 and 1 GB RAM so you should be fine there, but like SleepWalkerX says you might want to save up for at least a 6800 if not a 6800GT. Prices are slowly falling, I see Hot Deals in the $300-350 range for a GT now instead of $350-450. Wait a month and you might get a 6800GT for around $300 with Doom3.