Is it possible to have 768MB RAM on a computer?(eg. one 512MB and one 256MB card).

I know its possible to use 256/512/1024MB.

I tried 768MB in my PC but it wont boot up. Im guessing i have to get another 512MB RAM.

It's possible. You might want to try one stick at a time to see if any are faulty. Also, make sure you're using relaxed RAM timings (i.e. higher, like 3-4-4-8 for starters), in case the new stick can't handle the timings of the old one. They are both rated to run at whatever speed you've set them to right?

I’ve tested it and it’s not faulty. I’ve checked the timings and every thing but it still won’t boot up. My motherboard supports dual channel memory up to 4GB so maybe it has something to do with which slots I put the memory in. I’ll have to experiment.

change your motherboard settings to disable duel channel support. that should fix it cuz you arent running duel channel

What motherboard do you have? What make/model RAM modules do you have?

Its an Extreme K8 motherboard (i think) with DDR RAM

If the modules aren't the same brand and model line, then it's most likely the two modules are conflicting with each other, and won't be usable together. But reset CMOS first and see if they work OK afterwards.

Yea they are different brands. Ill have a play around with the cmos tomorrow and see if i can get it working...