As above said triple channel memory is not compatible with P55. You would have to go with X58 to get it, however there are barely any performance increases with it for anything. There is no Video Editing (I think) but it is still relevant:http://www.tomshardware.co.uk/Intel-Core-i7-Nehalem,review-31404-13.html
From this it looks like you should not go with an X58 system. However lets look at some performance charts:http://www.tomshardware.com/charts/2009-desktop-cpu-charts-update-1/Adobe-Premiere-Pro-CS4,1404.html
From this you can see that the i5 750 gets eat by the i7s and the X6s. So what should go with? As you can see the i7s are really powerful however they are also expensive. However take a look at the 1055T it delivers a massive performance increase over the i5 750 yet the build should be similar in price. The i7s increase in performance over it is not high and I'd say not worth the extra money.
Also AM3 will have support for AMDs Bulldozer CPUs, so upgrading would be as simple as smacking in another CPU. Contrary to Intel where they are discontinuing X58.
Conclusion: Get an X6 1055T instead of the i5.
Hope that helps.