Judging by your shopping list, you will need the new motherboard indicated. As a penny piching excercise, you might see if your modem will upgrade to the V92 status, although unless your ISP supports this, it will be useless. In your position, I would choose my processor 1st and then match the other components to the supporting motherboard. No point in purchasing items that the motherboard doesn't support. If you intend to keep the System box, check the power output, rating and amount of outlets, of the power supply also that a new form motherboard will physically fit.
Keeping your installation lean and clean, will help autocad run. When I used dial-up, I used Tiscali and found it reliable. Hope that gives you a start.