As soon as you either:
1. Resign yourself to a 5900XT instead.
2. Splurge for a 6800.
3. Find a 6800LE online somehow (maybe mail order from Kazakhstan).
4. Blow your savings on an LGA775 motherboard, processor, and DDR2
memory so you can buy the 6600GT PCIe.
6600GT aren't scheduled to come out in AGP format for another two months or so. If you're lucky they'll be available before Christmas but you shouldn't count on it. Add a little more to your budget & get a 6800 like Godataloss suggested.
It would seem the boards are ready, they just need to get supplies up & start shipping them.
On that picture you can see the PCI(e)-to-AGP bridge right next to the 6600GT chip. Also, contrary to PCI(e) card, the AGP version will need a molex power connector as an AGP slot cannot provide as much power as a PCI(e) slot can.
I'm also waiting for the 6600GT AGP. Its a great deal like the Ti4200 was for the GeForce 4 series. The only thing I dont like about the card is that they leave the SLI ability on the AGP cards even though it cant be used.