I'd go with a 8800GTS unless you are limited by your budget then start to drop down to something you can afford. To me the cost to go to the 9800GTX isn't worth it unless you have the money to burn. The 8800GTS will overclock WAY past the stock clock of the 9800GTX, but the memory on the GTS is basically limited to the stock speed or slightly higher. I believe the 9800GTX has better memory. Other than that there are a few little features that you will probably never use like tri-sli.
8800GTS is the best buy
They are all basically the same card, its just a reference card sold by different manufacturers. Some offer games with them and extra adapters, better warranties and what not. What are the best priced GTS's avaliable to you?