I was wondering if an 2005 model Athlon x2 4000+ would fit into a AM3+ motherboard. I am planning to start with an Athlon and eventually upgrade to either a phenom x6 or an FX chip.
Actually it won't. Even if he had an AM2 CPU the AM3+ boards require DDR3. All AM2 CPUs only have a DDR2 memory controller. Might get it to work on an AM2+ board. But all AM3 and AM3+ boards won't work.
No. Its, as said, is a AM@ CPU which only has a DDR2 memory controller. The Phenom IIs and Athlon IIs are AM3 but also have a DDR2 controller and can do either AM2+ or AM3/AM3+ but not AM2.
Your CPU is too old, you should get a new one. Either a AMD APU or a Intel Core i3. The performance difference will be instantly insane.