What can I say about the ASUS eXtreme N6600GT? The card is an excellent performer for the cost. An additional bonus is that these cards can SLI later, if you have a SLI motherboard and AMD Athlon 64 system. In some cases, the combination of two 6600GTs in SLI mode will outperform the equivalent high-end card like a 6800GT.
Of course, there is always the problem of something better is always around the corner. ATI's competition to the 6600GT, the X800XL has recently been released. Unfortunately, I cannot find this card online for anywhere near the MSRP of the 6600GT. Currently, the X800XL is $100 more leaving the market open for the 6600GT.
The bottom line on the 6600GT from ASUS is this; the 6600GT is an excellently performing video card for the money at this moment. NVIDIA has not announced a mid-range card to replace it for the spring. If you are looking to purchase a $200 video card, the 6600GT is a good choice.