Why would you not buy another gtx260, if ud buy another 4870. I'd seriously reccomend going with the 9800GTX+ in SLI mode. give that a looksy
Here is kind of an old thread but it compares SLI and Crossfire
3DMarks: Two HD 3870 cards are definitely more effective than two 8800GT cards in all Futuremark tests.This extra efficiency also makes the Crossfire post higher scores than two 8800GT cards in SLI making AMD the clear winner.
CoH - Opposing Fronts: Moving on to our first game, the SLI setup has a clear advantage over the Crossfire setup posting higher efficiency and thus higher scores as the 8800GT is faster than the HD3870 in single card mode.
Crysis: Crysis prefers nVidia cards in single and SLI modes. The efficiency of two 8800GTs in SLI is no match for Crossfire in Crysis making nVidia the card of choice for this game.
ET- Quake Wars: The Quake Engine scales very well under SLI as well as Crossfire tilting slightly in favor of Crossfire. However, the slight advantage for AMD is not enough to match the raw power of the 8800GT which posts much higher scores.
F.E.A.R. Like Quake Wards, F.E.A.R. also scales well under both SLI and Crossfire setups, however, it tilts slightly in SLI's favor. That, combined with the higher scores of 8800GT make it a faster on nVidia's GPUs.
Half Life 2- Episode 2: No real surprises here as AMD has been heavily promoting Half Life 2 under their camp and although the differences are higher at lower resoluiotn/settings, the efficiency between SLI and Crossfire is pretty close at 1920x1200 with 4X AA. Because of this, the Crossfire setup loses at this particular resolution and detail but manages to keep up or takeover the nVidia in other tests thus making it the winner.
Lost Planet - Extreme Condition: Again, Crossfire proves to me no match for SLI which posts much higher efficiencies across all resolutions/details. Adding to that the raw horsepower provided by single cards, Lost Planet is certainly a game you'd want to play on the 8800GT.
S.T.A.L.K.E.R.: Things take a turn in Crossfire's favor with S.T.A.L.K.E.R. where it posts higher efficiencies, especially towards the lower resolution and details. Unfortunately, this still doesn't allow the Crossfire setup to post faster scores than the SLI setup.
Unreal Tournament 3: Again, we see Crossfire posting higher efficiency at lower resolution/details and this time around, the difference is enough to make it faster than an SLI setup at lower resolution/details but as we crank these two up, SLI manages to take over.
Word in Conflict: Ending up our games, Crossfire takes the lead in efficiency which also makes it post higher scores than an SLI setup even at high resolution/details.
As you can see, when the settings are cranked up and the resolution is, SLI outperforms Crossfire.
