will my system cope with sonar professional?

It probably depends on how you intend to work with Sonar.  You might get lucky like me.
 
I run Platinum on my desktop DAW, as well as a 2nd copy on my laptop, which just happens to be an Acer Aspire as well.
 
I didn't buy this laptop for audio, just as a mobile office.  But decided to try it out anyway, and apparently just got lucky with this laptop!
 
This thing runs Sonar well, within it's 4GB memory limits.  Low latency as far as real-time audio. 
 
Recommend you download and run the free LatencyMon to check if your laptop is suitable.  http://www.resplendence.com/latencymon
 
I just use the Acer to play virtual instruments when I am traveling, so haven't tried recording any audio.  Using a USB external MIDI keyboard, the soft synths run well without latency or audio dropouts.
 
I use the on-board audio for playback only, but this setup would need an external pro audio interface if recording external audio such as a guitar or microphone (for pre-amps and instrument level connectors).  You would probably get better sound output with an audio interface as well, but mine sounds OK through my Sennheiser headphones.
 
My Acer Aspire E5-571-588M Laptop Specs:
 
Intel® Processor Type Core™ i5 mobile dual core/hypertheaded (4 virtual cores)
 
4th Gen Processor Model i5-4210U
 
Processor Speed 1.70 GHz Cache 3 MB 64-bit Processing (turbo to 2.7 Ghz)
 
Standard Memory 4 GB
 
Maximum Memory 16 GB
 
Memory Technology DDR3L SDRAM
 
Number of Total Memory Slots 2
 
Hard Drive Capacity 500 GB HDD
 
Optical Drive Type No
 
Screen Size 15.6" LCD Aspect Ratio 16:9 Screen Mode HD Screen Resolution 1366 x 768
 
Graphics Controller Manufacturer Intel® Graphics Memory Technology
 
 
 

Another thing I had to do on the laptop was to install ASIO4ALL to get the low latency plugin performance.  The stock Windows audio drivers won't cut it at all.
 
The alternative to ASIO4ALL is to at least get a compact 2 in/2 out USB audio interface that comes with factory ASIO drivers.  This will enable Sonar and the plugins to run at the desired audio buffer size.
 
Good thing you have a 23" screen, that's close to my DAW's monitor size.  The 1366 x 768 on my laptop is a little bit cramped on some VST plugin UI's.
 
Good luck! 

Any computer, no matter how powerful, can be maxxed out if you try. It's not a question of whether your laptop can handle SONAR (it can, easily), but how far you intend to push it.
 
I think you'll find an i5 with 8GB will do just about anything you want. If you do bump into a hardware bottleneck, it'll most likely be I/O because of your single medium-speed hard drive. Multiple large sampled instruments could be a problem, so if you plan on orchestrating movie soundtracks you'll be doing a lot of freezing. But if you're recording a band, singer/songwriter ballads or making electronica with soft synths, then you'll have to work really hard to max out that laptop.
 
P.S. I, too, have that same laptop. I use it for an even more demanding application, which is hosting virtual instruments for live performance. It easily handles even heavy-duty instruments such as Omnisphere and Keyscape.