You have a few hurdles here!
First you are limited on what CPU's will even work in your system. Jumping from an i5 to an i7 makes sense but upping the GHz really doesn't as its not that big a gain (i5 2.5 to 2.7GHz). You are locked in with the Sandy Bridge Core i7 2.7GHz (2600S) So before you go crazy consider that.
GPU is also a bit of a challenge to! You need to use Apple programed MXM GPU boards as they have custom firmware unless you have the skills to program the EEROM and the drivers also are a factor while you could put in a more powerful board the video driver will only run in compatibility mode so the extras are not active. Bottom line you're stuck with either the AMD Radeon HD 6750M or Radeon HD 6770M boards. And now the real kicker finding one that works! I would strongly recommend doing this task alone! If you don't and did every thing else you'll be pulling your hair out not knowing if it was bad or if you made a mistake in something else.
The hard drive is the last piece if you've going that deep why don't you just install the dual drive Installing iMac Intel 21.5" EMC 2428 Dual Drive Kit Then make it your primary boot drive and make your HDD a data only drive. That way you gain the performance of dual SATA I/O channels! If you just want the one drive don't forget you need the adapter frame for the smaller SSD.