as far as the 750 it shouldn't be a problem, depending on how many you install. Reports came in that if you used 4 of them and went over the Mac Pro's 2TB storage limit, it took a massive performance hit.
From Wikipedia -"The Power Mac G5 was introduced with three models, sharing the same physical case, but differing in features and performance. The 1.6 GHz model shipped with 256 megabytes (MB) of RAM, an 80 GB hard drive, and could employ a maximum of 4 GB of RAM. The 1.8 and dual-processor 2.0 GHz models shipped with 512 MB of RAM, and could employ a maximum of 8 gigabytes (GB) of RAM. The dual-processor model also included an ATI Radeon 9600 graphics card."