I did answer your question. Maybe I went into too much detail and you got confused.
*READ YOUR MOTHERBOARD MANUAL, PARTICULARLY THE GRAPH SHOWING CONNECTIVITY.
**You can NOT connect to the same DVI output and get a different signal to each monitor.
DVI#1:
connects to Monitor #1 (DVI->DVI cable)
HDMI#1:
connects to Monitor #2 (HDMI->DVI cable)
Had you been more specific it would have been easier. The HD5450 card I saw had a DVI, VGA and HDMI output.
To answer your questions:
1. "worked fine" ? Really? I can't understand how you got a different signal on both monitors using the same output..
2. YES. Install the full driver + ATI Catalyst software. Go to AMD's main site:
http://support.amd.com/us/gpudownload/Pages/index.aspx
SUMMARY:
Hardware:
It's my understanding that all you need is an HDMI->DVI cable.
Software:
Install and open "Catalyst Control Center" (link in the System Tray) and open "Desktop Management." You will assign Monitor #2 as the Extended monitor.
*Do note that if your second monitor is OFF you should disable it in the CCC. For example, if you dragged a program over to the extended monitor it's still there if you turn OFF the extended monitor, you just can't see it. Also, your CURSOR tends to get lost; if you turn the extended monitor OFF in CCC the cursor can't leave the main monitor.