You could always try ebay as a last resort if you really want the 6600GT.
Or you could wait a while, come up with some money, and pay the difference between the price of the card and the amount of money your mom is willing to spend.
The 9600 Pro or XT are both really good value right now for their performance though.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w