If you want, you can replace the i3 you have with the i5, especially for Shogun, and games with lots of graphics.
I usually recommend instead of doing like a massive upgrade to do one part at a time, so you dont buy things you dont need. A Sandy Bridge i5 would be a good choice.
The GT545 isnt the fastest, but it isnt the slowest thing out there either. I'd consider buying it off of you if you are looking to sell it since I need one for a spare computer.