Performance wise:
GX2 < 5870 < GTX295
However, 5870 = 1 GPU, GTX295 = 2 GPUs, so the price of 5870 is $100 cheaper,
so overall 5870 > GTX295
Then again, a 9800GX2 is the performance of 2 9800GTs in SLi, and costs around ~$140 on ebay...which is not too bad.