I'm usually pretty good about haggling. I think the most important thing is emotionally distancing yourself from the item, weather that be a car, a house, or a computer. I REALLY want a new Ti, real bad! But I play tricks on myself before I go in the store.
First, I tell myself that I've got a perfectly good working G4/533 that serves most of my needs. Then I remind myself that $400 dollars off basically means that I'm not paying tax and giving myself a RAM upgrade. And lastly, I remind myself that (unlike the Apple store and smaller Mac retailers) CompUSA is a high volume retailer and some store managers are willing to sell you a machine at cost to keep product moving. It's better to take even money now than to lose money down the line.
BTW - CompUSA is matching my offer. $400 off the cost of a new 800MHz Ti if I buy it today. I'm still considering.
Do you think I could get them to go any lower?