The term "shared RAM" usually implies that the video chip borrows RAM from the system. 288 is a strange number, but it sounds like there is 288mb of built in system RAM...like a DIMM soldered to the board which both the CPU and the video chipset can access...typically its dynamically allocated..you usually get about 32mb by default and the the video chip can request more as needed.

Again, since 288 is such an odd sounding number, I'm not entirely sure what is going on with the PC you are looking at...But 288 minus 32 is 256...so if the video chipset uses 32mb then you have 256 built in for the system...it probably has 2 open DIMM slots then where you can add more...maybe it comes loaded with 1 512 and 1 265 (unlikely though) which would give you 1 gb total....

With the Mac you get dedicated memory for the video. The video chip gets its own RAM chip, either 64 or 128mb...its not shared at all. Its that simple. There is usually a performance bonus to have dedicated RAM both for the system and the video subset.