50% discount? As in... $125?
Hell yes, buy it.
At the very least you can sell it for $250 on ebay when the GT300 is released.
You should see a rather large improvement in just about every game. Especially shader-heavy games and games with Physx.
Here is a basic comparison between the two cards:
8800GTS - 128 shader processors, 10.2Mpix\41.6Mtex fillrate, 41.6Gb\sec bandwidth
GTX 275 - 240 shader processors, 20.2Mpix\50.6Mtex fillrate, 127Gb\sec bandwidth
Quite a difference.