Let's say I have more money than sense. (4870x2/4890)

So I looked at some (read "one") benchmark's numbers, and have decided that the 4870x2 seems to be a more powerful card, BUT, it's basically two 4870s welded together, aye? Does that mean I can only crossfire two of them? In which case, making it better to go for a 4890, for future-proofing reasons? Or, say, I have too much money, and I can buy 3 4890s now.

(Got a DFI Lanparty DK M2RSH mobo, only 3 PCI-e x16 slots.)

TL;DR, I can has 3+ 4870x2s?

First of all what games do you play ?
What Res do you play them ?

Personally I would never get a X2 card...if something goes wrong with one (happened to me) your SOL.
Looking at benchmarks is fine but you really want to look at Real World Performance
LOOKING HERE You will see the 4870's win in benches but the GTX 280's win in FPS....in certain games that is.
But take all reviews with a grain of salt.
Let's not forget that EVGA/XFX/BFG all offer lifetime warranties too...hard to beat that

Not to mention I see that you fold 4 Dev...The Nvidias blow away ATI in PPD

Dev Hardware Forums > HARDWARE > ATI Video Card > Let's say I have more money than sense. (4870x2/4890)

Sorry man, I don't swing nVidia.

well you can see here how the HD 4890 compares to the HD 4870X2 spec wise, but the real world performance is going to depend on how hard you are going to push a card like either of these. As PaPa_Capinuraz noted, the games you play and resolution you play them at is absolutely critical. There is no point spending the money (whether you have it or not) on something that will be underutilized.

The bottom line is basically neither of these should be options on the table unless you are running a monitor capable of 1920x1200 (or 1080p) or above. For even the most demanding game today, you wont find an HD 4850 or HD 4870 512 struggling to keep up with 1680x1050 on those nice 20-22" panels. And at this point you are looking at the $100 range instead of the $200-$400 range, without sacrificing any gaming performance for that resolution.

To go with dual GPUs, triple..or even quad, you really have to take a lot of factors into consideration to get the most out of your purchase, and you cant be strictly set to one company or another. Some games simply scale better on ATI cards than on Nvidia cards, and vice versa. Also note the strain on your processor when scaling beyond 2 gpus starts to be come a worry, as the drivers need to be working much harder to keep those cards fed with data. So it quickly becomes worth while to have either a high clocked dual core cpu, or start looking at how much you can squeeze from a moderately clocked quad core. And beyond that another step is the OS, a lot of people still run XP but Vista (or 7) is what you really want for multi gpu operation because the OS is simply better suited to it, and being able to utilize DirectX10 (and DX11 in 7) will give you better graphics, and in most cases DX10 performance scales better than DX9.

On the topic of future proofing, I tend to be the hardest critic on that, I honestly dont believe its viable. The product cycles for modern high end equipment are shrinking to 8-12 months instead of 12-14 months as in the past, and sometimes we may see a premium version launch less than 8 months from its predecessor. When it comes to high end gaming, you can typically get 1-2 years out of a high end graphics card depending on how many new and highly demanding games you add to your collection. And since every product cycle tends to bring new refinements to performance, energy efficiency, and price, its just not worth it to say "ill buy a $500 card now because it will last me 5x longer than a $100 card", because it just isnt true. It may last you that long, but with every passing quarter you get better options that bring newer technology and all the advances that come with it (power, efficiency, and price).

My motto is buy the performance you need, when you need it. Don't buy the best card on the market today because it might last you a bit longer than a lesser card. Remember that the HD 4870x2 for example is built on the exact same technology as the HD 4830, HD 4850, HD 4870..they all use the exact same RV770 gpu (the HD 4830 simply has reduced SIMD count). The RV770 was just refined once again and has become the RV790 used in the HD 4890. Because all these cards rely on the same base technology, you can see the performance curve pretty much effect all the cards in their segment...When a HD 4870 is no longer fast enough to play a game, the HD 4870 will only just be fast enough, so why not just buy the cheaper card now and replace it in a year or two when a game comes along you actually need the extra power to run.