From my experience, AGP 4x & 8x wont give, if any, real world improvement on photoshop. The bottleneck is at the CPU, RAM & HD speed.
Last time I changed my office PC video card to radeon 8500 (the fastest at tat time), the improvement is still marginally small compared to the on-board vga. Btw, the cpu is PIII 700Mhz with 512Mb PC133 ram.
But putting the same video card on my house pc running 1.8A@3.0Ghz with 512Mb DDR400, its a different story...Blazingly fast...
Unless u wanna play the latest 3D games, ur Matrox card is good enough for most image editing...Upgrade ur other components instead...
Just my 0.02b worth