My suggestion is to change the current iMac so that it opens easily for upgrades such as RAM, HD, GPU and maybe the CPU.
That's not the concept behind an iMac (or the original Macintosh). The concept is, that people don't have to worry about upgradablilty. You buy it, it works just great for a couple of years and then you buy a new one. It's not about swapping the CPU or GPU by yourself. I mean, if you think of the targeted consumer of an iMac, it's not what they can, should or want to do.