If you really want to use a PC so you can easily upgrade components one by one, then for sure do NOT buy a Dell. Build your own from the ground up.
Good luck getting a Dell to work correctly with an upgraded processor. The motherboard BIOS may be locked to a specific processor or processor family and there's no way Dell will support a modified machine. And likely you won't be able to fit a new, non-Dell motherboard into the case if you want to swap that some day, too.
You're better off building your own.
That said, people use Macs for more than just the hardware it contains. For many, Mac OS X is a superior way of getting things done. If that doesn't interest you, and all you care about is bang for the buck and can tolerate Windows - or high-level gaming is now a priority - then no one is stopping you.
Or, add some RAM to your iMac and enjoy it. The i3 isn't a professional-level processor but it's perfectly sufficient for day-to-day desktop duties.