iMac 27 vs MBP 13?

So I have a mid 2012 Macbook pro 13" with i5, and I've recently upgraded to 8gigs of Ram. What I noticed is that my graphics unit: Intel HD Graphics 4000 went up to 1536 MB from 1024MB. How is that possible just by upping the RAM? Also, I've been editing videos (CS6 premiere) on the same macbook and i'm now planning to get an iMac 27 instead. I'm looking at older versions (2009, 2011) and I've I noticed is that they mainly come with graphics card with 512 MB (eg. ATI HD 4850) Does that mean that they will perform worse than my macbook pro? For some reason I've always thought that iMac's generally are better for PS and Premiere Pro and such pro design apps. So my questions are:

Question 1: Is that possible that my gpu went up to 1536 MB from 1024MB just by upping the RAM?

Question 2: Is it the size (megabytes) that counts in the graphic performance? Will iMac's with smaller size (in megabytes) gpu's perform worse or better than my current MBP?

Question 3: Which older iMac is a good model for using CS6 premiere pro? (budget pretty tight)

Thanks a lot in advance!