Having flip flopped back and forth over the years with nVidia and ATI, think I'm going to dump my 8800GTX OC for two 4870's on an ASUS Rampage Formula X48 (DDR2) motherboard.
 
Decision is based on:
 
1.  GTX 280 poor performance in FSX
2.  GTX 280 65nm and considerable heat and fan noise
3.  Two 4870's come to about the same price (maybe a tad less) than one GTX 280
4.  Drivers for the 4870 seem more mature
5.  DX 10.1 support (spec validates AA capabilities more than anything)
 
The big question is to go DDR3 or stay with DDR2 motherboards.  Replacing my aged P35 but I've got two 4GB Corsair DDR2 that are running extremely well (read reliable) with my Intel QX9650 @ 3.6 Ghz.
 
Please this is not a fan boy thread, I don't really care who is on top of what.  GTX 280 is a great single card solution it just doesn't work well in FSX which is important to me.  Unfortunately I don't really know how well the 4870 works in FSX either.
 
Real question is, DDR3 motherboard or DDR2 motherboard?
 

If you're willing to drop 600$ on video cards for the sake of "futureproofing" and throw away/sell your previous motherboard/gpu, then I think it is reasonable to take the additional expense of ddr3 if you do in fact believe in future proofing
 

 
If I was in your situation I would've SLI-ed another 8800 GTX, but thats just me. I think you would find 8800GTX SLI would perform in the same region as the gtx280 (given that 8800 GTS g92 does, and the 4870 CF is only marginally above that) but that is assuming you have  an SLI board.
 
Look, I would seriously consider sticking with a single 4870 if you do infact plan to do this changeoever - and then if you want more power get a 4870x2 to CF with or even just add another 4870 later after the word is out on the 4870x2
 
The 4870CF is only just fair scaling right now - 1 gpu has great performance but 2 gpus is only a 30% increase in games where you would have a performance issue to begin with.. and the 4870x2 is supposed to be much better technology for that matter.

Congrats, you have more money than you need.

Don't think I mentioned anything about wanting to "futureproof"?  Just curious if I'll see any gains from DDR2 to DDR3.  Nothing is futureproof and never will be, not in this industry, hell, not even in real estate.
 
FSX doesn't really show any improved FPS from either SLI or Crossfire, so in this case would a single 4870 out perform the 8800GTX in FSX.  The reason for getting two is because it would appear 4870 on the right motherboard does indeed show some impressive performance on the more recent games that do support Crossfire and SLI.  I'm not a big fan of Crysis, but it's fun to wonder that leafy universe every once in a while.
 
Why is it someone always has to post a money response??  Envy?  Insecurity?  Pissed cause they haven't figured out to earn "more money than you need"?  What is the relevance?
 
If folks don't know, just don't post, peanut galler responses are just a waste of your time and anyone elses time reading this.
 

Well if you already know what you're looking for, why are you asking others? You could've truncated your entire post to:
 

 
Which any person in their right mind is going to tell you: NO, its a waste of money for no performance increase and you should only go DDR3 with the future in mind.

I think it comes down to how much money you want to spend. I agree with the OP that future proofing is impossible. I would go with DDR2 just because of the perf. between the two. You could pick up a set of the new OCZ Flex II 2x2 gig kit. Its @ 1200 mhz. I would prefer the P45 over the X48 just for better overclocking. I am not sure if you can get DDR2 with he P45 though. I am at work and can’t check.  

 
Congrats Deuche, you seem to have more money than you need. In fact, anyone who has the time and capacity to post on the internet about graphics cards are probably living above subsistence levels, so I guess everyone here has more money than they actually have a need for. Anyways Deuche, maybe he used credit.

I always use Credit -- so long as no interest is involved.    -- besides, with AMEX I get a ton of money back come year end, if possible, I try to buy everything with my AMEX (even food) cause it pays me back so well and I get about 30 days free roaming.  Now if you mean do I use credit and then not pay it off, NO, never do that because that is a waste of money.  If I'm gonna waste money, I prefer the tangible
 
Since I already have some good DDR2 nuggest, I'll stick with DDR2.   By the time Intel's next processor comes out, I'll need a new MB anyway so will do the DDR3 thing then.
 
Finally found some data on the 4870 and FSX, it's a little better than the 8800GTX Ultra when 4XAA is used.
 
Yes, perhaps I should have restated the question DDR2 or DDR3?  But I thought I'd leave the door open for any other input...and thank you for your input.