If u still want SLI, i say go for 2 8800GTX instead of 1 9800GX2 but plz read the FAQ and then consider
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XPS M1730,IntelCore2Duo T9300@2.5GHZ,2x8800MGTX in SLI,4GB DDR2 667 RAM,DUAL 250GB 7200 RPM,VISTA HP with SP1,17" with 1920x1200 resolution
I would wait a week or two for the new cards from ATi and Nvidia to come out with official benchmarks.
You might be able to sell your current card and make enough money to buy the new ATi .. It might do 20 - 40% better than what you have now all in one gpu.