When the reviews were done for "HD3870 X2 versus HD3870 512MB Crossfire", do you think most reviewers used an X38 motherboard that has full 16x speeds for both PCIe2.0 slots? As you might know, that chip that links the two GPUs on the 3870X2 is only PCIe1.1. So when both single 3870s are running at full PCIe2.0, and in crossfire, I find it odd that the 3870X2 would outperform two 3870s in crossfire. After all the specs are the same (640 (320 x2) stream processing units and 512 (2 x 256-bit) memory interfaces). Also, two 3870s have 1GB DDR4 vs 1GB DDR3 on the X2.
Correct me if I'm wrong, but the core speed should be slightly higher (825 vs 775 I think?), and the memory slightly lower (900 instead of 1125mhz I think?) on the 3870X2. Also, the X2 is using GDDR3 with differing latencies than the GDDR4 3870 which may slighty offset the memory speed deficit ever so slightly. I think these would skew any direct comparison until they are tested at the same clocks.
I'd say the answer for anyone without a Crossfire board (like myself) is a 3870x2. The card's "out for delivery" today, the new AMD single slot motherboard for a stopgap should be here tomorrow, and then I can transfer my 4600+ from that lousy 405 chipset Nvidia board where the documentation says it does not support ATI cards.
Anyways, if you have a nice AMD or Intel Crossfire board and one 3870, the least expensive choice would be a second. After all, R700 is around 6 months away and R770 within a year of that.
If you're getting a Crossfire board and can afford the 3870x2, I'd go that route instead of two 3870's. The reason is that 3870 in Crossfire won't get much better, but you can always add another 3870x2 or R700 down the line.
The main reason I went 3870x2 is that it's Crossfire performance without the hassles, and when the 780g boards with power saving mode is out; plus 45nm Phenom, then I'll switch from the 4600+ and MSI 690v.
If the MSI K9N Nvidia 405 board had accepted ATI cards, then I would have kept it until power saving made switching worthwhile. Sadly, the documentation says:
(Due to the specification of the chipset, ATI X8xx, X7xx, X5xx, and X3xx series graphic card will not be supported.)
The manual added the x1000 series to the list, which is why I ordered a 7600gs for it originally.
MSI's tech support said to get an Nvidia card for an Nvidia board, but I decided I liked the 3800 series better then the 8800, so I had to switch boards for my gaming PC.
Our other Athlon X2 PC's have HDMI 690G's, but I didn't need that for a two month solution. Nor did I want to use the old P4 630 on an X200 board for such a powerful card.
Isn't it great that both Nvidia and AMD are coming out with motherboards that support hybrid SLI or Crossfire for low end cards and power saving mode for high end? Who needs the power draw of a 3870x2 or 8800gts at idle when posting at Tom's Hardware? It's just too bad the 780G isn't out yet, or I'd have ordered that instead of the 690v.