I am sure the Mini will be faster.
Unless the mini is offered with a quad core CPU, it ain't gonna happen.
http://www.videocardbenchmark.net/gpu_list.phpeven the best graphics card you can get for a G5 is worse than the 320m in a Mac Mini (unfortunate but true).
GeForce 320M 330th
Radeon X1900 GT 134th
The 320m scores 309 vs the X1900's 791. So clearly you've got a major lack of knowledge on the subject.
Even the "Intel HD" GPU on the i-series CPU that will be used in the next mini scores only slightly better than the 320m (Intel HD 326score, 314th), but is still only a fraction of the power of the X1900.