Yeah I had been considering laptops also, as we have kids and this works well with clearing the unit away at times, but there's all sorts of issues with 2nd hand units, as lots of mobile equivalents are dual core instead of quad core in desktop PC's. i5 etc doesn't always mean quad core when you go to the mobile world etc.
Obviously it's been a while since I last came up to speed with CPU's etc!
dman: You'll always have to spend more on a laptop to get the equivalent power of a desktop.
Plus you've got more issues with heat, and often less parts that can be swapped out if they fail. Often GPU is fixed on the motherboard etc. They do offer flexibility though, so it's a tradeoff. As per the original post though, I'm after bang for buck, so a laptop doesn't offer the best of that compared to a desktop. Will keep looking...
Have an SSD coming since I happened to win a trademe auction for one quite unexpectedly.
Have ditched the ATI Radeon HD 5770 video card for a nvidia equivalent, GTX 650, to be more compatible with Adobe Premiere CS5 or CS6. Seems just as powerful as the old one, but uses less power, so expecting I won't have to upgrade the power supply.
Just waiting for parts to arrive and then will get to installing the SSD and software. Thanks to all for the comment/ideas/soundboard to bounce ideas off etc.