Two 9800GTX in SLI beats one 9800GX2. However, the one 9800GX2 is more energy efficient. Three 9800GTX in Tri-SLI is actually slower than two 9800GX2 in Quad-SLI, especially in games that handle it better. At high resolutions, the Quad-SLI can get over 10 frames higher in COD4.
The thing is, these are both at stock speeds; I have no idea for overclocked. Clearly, the 9800GTX overclocks substantially more. However, I don't really think it would surpass it, rather than just close the gap.
Overall though, three 9800GTXs is cheaper, but only the 680i, 780i, and 790i can use it. Two 9800GX2s would cost more, but any SLI capable board can use them.