That is where most of my confusion comes from I guess. The 5450 only has 512MB, but how much of the system memory does the integrated 4250 get to use? And since I would have, what I consider, an abundance of RAM at 8GB, I don't think I mind sharing with the GPU.
The only time you need more that 512 MB of ram for graphics is when you are driving high resolution( above 1920x1080) displays with large texture files. Also the ram on a discrete card is faster(ddr5) than system memory(ddr3). the discrete card has its own memory controller, and doesn't have to share it with the rest of the system as the integrated graphics does. So, yes, theoretically, integrated graphics has more available ram than the discrete card, but unless you are using it, it won't make a difference.
I would rather have a faster card with less ram than a slower solution with more ram.