You need to upgrade the RAM on your MacBook pro. I think it will take 16GB.
It's always cheaper having to upgrade one system instead of two. So I'll just stick with the high-end laptop plus external display model. If you want a Desktop that's really head and shoulder above your macbook pro you need to step up to Mac Pro or custom build high performance PC with a 6-core CPU, SSD array and serious graphic cards. The PC solution is about $3500 while the Mac solution is about $5500. Both of these systems will be obsolete in 3-4 years and worth as little as $500
so it's a cash burner.
Furthermore the retina screen for iMac isn't bad, but it's glossy and doesn't really stack up to the NEC displays that are specifically made for critical image processing. When you want to go beyond web posting and prepare your files for print/publication you will realize the value of such screen. It will serve you for many years to come
best