I'm messing with my matrox cards collection and I'm trying to test their overclock capabilities, here are my findings, all the matrox cards uses dividers and the clock between core and mem is ganged:

Matrox Mystique 618-02 Rev: A: original clock 60\90, no room for any overclock due to the 12ns memory.
Matrox Mystique Hybrid 644-03 Rev: A: It uses the new 220 ramdac plus the old 1064SG-D chip, the clock is the same as the original mystique but it uses 10ns memory which allowed me to overclock the card up to 76\115 (if cooled 80\120 can be reached but with some memory corruption).
Matrox Mystique 220 644-03 Rev: A: Same as above it uses the new 1164SG-A core which is the same as the old one, same overclock results 76\115 (if cooled 80\120 can be reached), there are rare versions with 8ns memory they probably allow to reach higher speeds, unlucky the price for those card is unreasonable.

Matrox Millennium 590-05 Rev: B: Factory speed is 66 mhz for both core and memory, I was able to overclock it to 74 mhz. For gaming is pretty useless due to the lacking of 3d features.
Matrox Millennium II 708-04 Rev: A: Newer chip and features on par with Mystique 220, not always faster quite the opposite, again the stock speed is 66 mhz overclockable only to 72 mhz due to the video ram limit.
Matrox Millennium II AGP 672-04 Rev: A: Same as the pci model, maximum clock is 72 mhz, faster due to the use of agp bus, it's superior to Matrox Mystique 220, but you need an agp capable motherboard, it's not worth.

Matrox G200 834-00 SDRAM: Cheaper version of G200 with sdram, despite of that the most funny to overclock, standard clock is 85\113, I was able to reach 110\147 it's probably possible to squeeze some other mhz, it uses 8ns sdram.
Matrox G250 873-03: Same as G200 sgram version but the core has been die shrinked and it's without heatsink, clock is higher: 105\140, overclock wise it's very disappointing, I tried to install a heatsink but I couldn't go higher than 110\147, the limit is the core, the card hangs if I try 115 core speed, it has been probably pushed to the limit already, it uses 5ns sgram which in theory supports the speed up to 200mhz.

Matrox G400 MAX 906-04 REV : B: I have two versions of the card, one is the older 906-01, the other the -04 rev b, the difference (thanks to the vogoners who told me) is that the rev b supports the agp 4x correctly, the original clock is 150/200 overclock wise they are almost the same (a couple of mhz better the rev b), I have been able to reach 165/220, some cards are able to reach an higher clock but being the max don't expect much overclock capabilities.
Matrox G450 971-0302: 32mb version with ddr memory, original clock is 111\167, I was able to reach the 127/191, it uses ddr memory and they are rated for 5.5 ns, not bad.
Matrox G450 PCI 7003-0301: again 32mb and ddr memory 115/144 , the original core clock is a bit faster compared to the agp version on the other hand the memory is clocked much lower, I was unable to user powerstrip to change the clock because after the new one was applied the card slowed down (I found the same issues with 2 different cards at the moment), the alternative is to overclock with pins. I was able to reach 143/179, but I'm sure there is some space for higher speed, I also tried to clock the card at 200 mhz memory wise but nope, it doesn't work, screen corruption appears just after.

Matrox G550 7012-03: the original clock is 133/166, there are two versions with different rated memory 5,5ns and 5ns, I have the 5,5ns and with powerstrip there is no head for any overclock as soon I change the speed even of 1 mhz the screen gets vertical banding and increasing glitches, I'm not sure if it's a software issue, I will try to overclock with pins but I believe the 5,5ns memory version is very limited.

I will update the topic if anyone is interested, please feel free to post your experiences too!