Yeah, it'd definitely involve spending more, but probably not as much as £1000, because I could sell my current MBP (2009, as it happens) and recoup some of the difference,I can't see into the future but I wouldn't do that if I were you, (which I kind of am). You'd end up spending about £1000 more for a less powerful machine with less memory, may well be portable but so is a 2009 macbook.
As for the power, I'd probably go for 16GB, because the upgrade price isn't as ridiculous as it used to be.
What's the power of the 650M like compared to the 6770M?