Well, if you don't mind going ATi, they are releasing a card perhaps in August (rumor has it anyway).
If you're gonna stick with Nvidia, if you have an el-cheapo card you can use until NV30 comes out (Sometime this fall), That would be the fastest card, keep in mind, its going to be pricey.
Its a matter of preference, If you have the money, and if you don't mind not playing any hard core games for a month or two after September 1st, I would go with Nvidia's new chip this fall, and see how it performs. If you must have something top of the line, and can't wait a month or two, I'd go with the ATi R300. Or Ti4400 (I would suggest this over the Ti4200) if you want just a nvidia card.