The coreclock of the 'normal' Matrox G400 is 125MHz, the memoryspeed is 150 MHz. G400 MAX has a 150MHz clockspeed and a 200MHz memoryspeed, but I assume you mean the 'normal' version. If you use G200clk try typing "g200clk /1 290". This setting should work (or else go a littlebit lower, like 280). With a PLL-clock of 290 you end up with a coreclock of 145MHz and a memoryspeed of 193MHz. That's almost MAX-speed! If it works that is. It worked with the card I tried it with, so good luck!
G200CLK is a freeware utility you can use to overclock your G400 card. Go to "WWW.MATROXUSERS.COM", then go to the downloads-section. There you can find it under 'utilities', it's called G200CLK 0.21 beta. If you are new to overclocking or don't know exactly what to do, post it here and I will explain it to you.