Holy shitfuck, how much are you paying for that?!

You rich, rich man.

As for the 32/64 bit Windows issue - what will you use the system for (apart from gaming)?

If you're primarily playing the newest games and checking your email, 64 bit windows is fine...

There are other issues with 64bit versions as well, but the real question is why would he want it? I'd assume he wants it for the additional RAM a 64bit system can offer, however no game requires 6GB of ram to run. The only time I went above 2GB is playing Crysis and unless you have a bunch of shit open you will probably never go above 3GB. I'd think it would be preferable just to get 4GB and stay in 32bit until you really need that extra ram. It's not an issue to put 1 or 2 more sticks in and it won't be anytime soon.

If he's running triple channel, he would need three of the same sticks, so it's either 1.5gb (3x512mb) or 3gb (3x1gb) if he wants to stay under 4gb.

I haven't run two 4870s so I don't know for sure. But since it's essentially a single card, there's no overhead of CrossFireX, so I doubt that you lose much performance.
Are you talking about 2x280 vs 4870X2? Because obviously those would be better (and twice the price tag too), since GTX 280 and 4870X2 are both their respective manufacturer's top of the line graphic cards. But if you're looking for a single card setup, then definitely go for 4870 X2, it's better and any benchmark shows you that. Check this one for example (comparison of 4870X2 vs GTX 280): http://www.madshrimps.be/?action=getarticle&articID=869

It's especially better under Vista, which you're getting, and it deals with AA much better. I found Crysis benchmarks very interesting:
http://www.madshrimps.be/?action=getarticle&number=9&artpage=3692&articID=869

4870X2 pretty much beats the 280 by a very large margin (twice as fast in some cases) in Vista and when you enable AA, while 280 is only marginally better in XP with no AA.

I have a question. Do you get ~200% performance with a 4870X2 or does it suffer the same sort of ~120-160% performance difference of running two 4870s in crossfire versus a single card?

You forgot the pricing on them. The 4870X2 is about 500$, and the GTX 280 is about 380$. Sure it's faster, but is it 120$ (~30%) faster? Doesn't look like it. GTX 280 still has best price-to-performance ratio from these two.

Two cards in SLI/Crossfire will almost never hit a 100% performance increase (I think you're doubling your percentages there!), and the average is around 50%. You have to factor in that some games won't fully utilize sli/xfire, lower resolutions/detail settings will see the lowest increase in speeds, and some games just won't work with it at all.