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Originally Posted by Dissonance
On the graphics card, after doing some quick skimming, it appears the 4870 beats the 9800x2 on every game but Crysis by a good amount. So the 4870 will save you a bit of $$ and be better  |
From the most comprehensive review I have seen, the 9800GX2 reliably out performs a single 4870 GPU.
AnandTech: The Radeon HD 4850 & 4870: AMD Wins at $199 and $299
A 9800GX2 is essentially two 9800GTX GPUs integrated into a single unit so that you don't have to use/have two PCIe X16 slots to SLI two single cards together. Now if you have two PCIe X16 slots, you will generally get better performance from two 4870s in Crossfire mode. The two 4870s will cost a bit more (~$150US total) than a single 9800GX2 using Newegg as a price guide. You would have to decide if the performance increase is worth the cost.
Some games only show only a couple frames per second (fps) increase, other show an ~40% improvement. That is the tricky aspect of both SLI & Crossfire solutions. Neither scale 100% in every application. It depends on the coding of the given application.
By going from one card to two, or a single 9800GTX to a 9800GX2, or a single 3870 to a 3870X2, you may experience a performance increase of ~80% to 0%. I have never seen an application gain a 100% increase in fps and sometimes applications do not support SLI/Crossfire at all so you gain nothing from having dual GPUs. It is rare these days though for developers to not recognize the potential for end users to utilize a dual card set-up so most applications will benefit to some degree from a dual GPU configuration.
So for best performance choose either one 9800GX2, or add ~$150US and go dual 4870s (assuming your mobo has two PCI Express X16 slots).