crwdns2893792:0crwdne2893792:0 Dan ,
Your options are limited here. Yes, people have upgraded their i5 to i7 CPU's and GPU with units that are compatible with the given model of your system. Review some of the iFixIt threads for some successes and failures. It does take more advanced skills to do this. Frankly, if you've never opened or serviced a few iMac's I wouldn't waste the time or the risks. I think you'll be better off getting a new or used system that meets your needs.