Firs off the 5450 is not a gaming card. the 4870 is the recommended ATI card or better for the gam and the X1950 ATI card plus the 4870 are both better cards. So I will link you to the hierarchy of the GPU's so you can see for your self.
When you chek scroll down untill you come to the first intell card on the right. Right next to it in the middle of the nVidia and intel is the AMD and you will see the 5450 then scroll back up untill you see the X1950.
You will see how far it is above it, it is. Then like said above you have to know if your PSU will handle a new card along with what type of CPU and RAM. That why it is good to list all that stuff when you ask a question.
I hope this has help and I wish you good luck on fixing your kids PC in order to play Diablo 3. I have 5 of my own and when they were little I know how it was now they get to buy stuff for there kids.
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
Diablo 3 Minimum System Specifications:
PC
OS: Windows® XP/Windows Vista®/Windows® 7 (Latest Service Packs) with DirectX® 9.0c
Processor: Intel Pentium® D 2.8 GHz or AMD AthlonTM 64 X2 4400+
Video: NVIDIA® GeForce® 7800 GT or ATI Radeon™ X1950 Pro or better
Diablo 3 Recommended System Specifications:
PC
OS: Windows Vista®/Windows® 7 (Latest Service Packs) | Processor: Intel® Core 2 Duo 2.4 GHz
or AMD AthlonTM 64 X2 5600+ 2.8 GHz | Memory: 2 GB RAM | Video: NVIDIA® GeForce® 260 or
ATI Radeon™ HD 4870 or better