I don't understand your question.
I think what you're asking is;
"Am I going to see a performance improvement between a machine with a 1.5ghz CPU with 1.5gb RAM machine and a machine with a 4.4ghz CPU with 4gb RAM machine"
You were running at 90%. Worst case scenario is you now can run three times as much stuff. Best case scenario, you don't just switch all your data over (and can just move projects over as/when needed) and can 'start fresh'. Worst case scenario - it sounds to me like you've done something majorly messy to get yourself in that situation - either leaky 3rd party software or using stock channel strip presets in Ableton? If so, why not find a way of working that suits you and your music rather than what suits Ableton's stock channel strips?
If you're asking whether you should wait and buy a MBP instead of a MB, here's a couple of links for you; MacBook Tech Specs MacBook Pro Tech Specs
I'm sure you can compare the (seemingly similar) figures yourself.
Buy the laptop with 512mb RAM (or as little as you can buy) and do a 3rd party RAM upgrade to save some coins - but a quick search would have told you that also.