The only qualification I would add to BDAqua's excellent response, is that performance can indeed be improved for certain applications, such as Photoshop.
Beside the graphics card upgrade, which is a given, performance of Photoshop is improved substantially with the addition of a dedicated, large-ish internal drive for Photoshop's scratch disk, e, g, a 160 GB or larger fast hard drive, or even a 140 GB0 or larger SSD,
In my case, I'm upgrading a 2.5GHz PowerMac(PPC) G5Quad with a mutant, flashed 550MHz nVidia GeForce 7800GTX, 1,700MHz graphics card with 512MB VRAM to drive dual high-end 22" CRTs, 16 GB of RAM (yes, you can observe a difference in the performance of Photoshop going from 12 GB to 16 GB of RAM), lots of internal and external hard drives, and an ATTO SCSI card to drive a tabloid-size transparency scanner and a film scanner. I do plan to get the SSD as soon as I get a Sonnet G5 Jive mounting system.
Cost-wise, I'm still coming in ahead of a new equivalent Mac Pro, and I'm retaining the ability to run thousands of dollars of irreplaceable custom software in Classic under Tiger 10.4.11.