The only numbers we know of the 7770M puts it slightly ahead of the 650M but at only 30W TDP. Either the 650M is lower TDP than a 550M or the 7770M is still better and faster. I guess the former.
AMD's 6770M (currently used in MBP) is believed to have a TDP of between 30W and 32W. Its replacement (the 7770M) has a TDP of 32W [
source].
Nvidia's - Fermi based - GT 550M/555M has a TDP of between 30W and 35W [
source], making it comparable (in TDP only) with the 6770M/7770M. The newer - Kepler based - GT 650M appears (and I'm dubious about these numbers) to have a TDP of 45W [
source]. If this is truly the case, the new MBP is more likely to have the 32W GT 640M (despite the rumours of a GT 650M).
There are 'processing power' numbers listed in the tables I've linked; from which you can infer ballpark-performance. Direct comparison of GFLOP numbers is not possible as Fermi was a very GPGPU focused architecture, whereas Kepler is gaming orientated (with relativly poor GPGPU performance). AMD's new GCN architecture (7770M and others) is more balanced (in terms of it's focus) than Nvidia's Kepler. GCN appears to have slightly worse gaming performance than Kepler, but better GPGPU performance (particularly OpenCL, which Apple are pushing).
Which GPU is best (and by how much) is very subjective. Five years ago, gaming was insignificant on OSX, but that's not true today. If you're after gaming performance, the GT 640M (with GDDR5) will likely be on par (if not slightly better) than the present AMD 6770M, whilst the GT 650M would provide a large increase in performance. Conversely, if you're GPGPU focused, it seems the GT 640M will be a large step backwards from AMD's 6770M, and even the GT 650M will struggle to match the present MBP's GPGPU performance.