Is a 4870 X2 worth it over these other options?

Yes, I think its worth it. Although your monitor/resolution would be helpful in making a decision.

4870x2 is expensive because it's the bleeding edge right now. People are willing to pay big $ to get the absolute best. In term of strict price/performance ratio, dual 4850 is definitely better.

Another reason for the price is, it's new. Latest generation hardware cost more. That's always been the trend. The best performance/value is always one generation lower. Look into "yesterday's best" if you want performance on the cheap.

Since 3000s ATI cards perform far too low compared to 4000s, let's look to Nvidia's side to illustrate this.

gtx280, $430:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
9800gx2, $285:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Performance comparison:
http://www.anandtech.com/video/showdoc.aspx?i=3341&p=13
http://www.anandtech.com/video/showdoc.aspx?i=3341&p=14
http://www.anandtech.com/video/showdoc.aspx?i=3341&p=15
http://www.anandtech.com/video/showdoc.aspx?i=3341&p=16
http://www.anandtech.com/video/showdoc.aspx?i=3341&p=17
http://www.anandtech.com/video/showdoc.aspx?i=3341&p=18
http://www.anandtech.com/video/showdoc.aspx?i=3341&p=19

Anyway, for both good performance and value, dual 4850 is the way to go if you already have the x48 motherboard. Graphics technology move fast, there's no such thing as "future-proofing," so get whatever is the best value at the moment.

750w not going to do it I think

750w is enough.
http://www.anandtech.com/video/showdoc.aspx?i=3341&p=22

Yea, 750w is enough for one 4870 X2. 750w could be enough for two if you only had to worry about watts. The problem you run into is 750w PSU's only carry 60A on the 12V rail (at least for single rail PSU's), which I don't think would be enough for two 4870 X2's and everything else combined. That's only from what I can read on google though, I can't find any official power consumption specs for the 4000 series cards. It would be nice to know though...

The 4800 series was a big improvement for ATI, but I still find that the 3800 series performs like a champ, particularly the 3870, as long as you aren't gaming at resolutions over 1650 x 1200.

Graphics card capability seems to be outpacing the graphics demand in games as of recently.....

3870 performs the same as a 9600gt, barely mid-range. It's not in the same league as what OP is looking for. "Champ" isn't the right word to describe it.

i can get the 4870x2 in the uk for £329, the 280 when it came out was £600, nvidia has always charged more for there high end