Please help me! I'm trying to install 2x 512mb DDR SDRAM + original 256mb.

System Information reads "Availible System Memory" correctly but "Physical Memory" is at 256/512 depending on whether I put them all/2x512 in. Not sure what to do - any help would be gratefully recieved.

Running XP and Jetway 866as-r motherboard.

Welcome to TechSpot

Are ALL your memory-chips the same? i.e. all PC2100 or all PC1600? You cannot and should never mix them
Did you try to boot up with each piece individually (3 boot-ups)?

Go to www.memtest386.com and D/L memtest. Run it overnight with all 3 pieces in, to determine if any memory is faulty.

All 3 are 2100, it's almost as if Windows can only see what's in the 1st slot. When I start-up it counts the ram correctly - so I presume all 3 must be working.

I'm mystified- this is the first time I've tried to upgrade, I followed all the instructions I was given- it's becoming a nightmare!!!

How about filling up your system specs section in your profile ? That way we know what hardware with the Ram.

I have a new processor ready to install (AMD 2400+) and I'm planning to install a new graphics card soon. I thought I'd start with the RAM (thinking it was the easiest!). The RAM I'm dealing with is Corsair 2100 the old is "TwinMos" 2100.

I've also update my profile if it helps.

Thanks

Two different brands of RAM, even of the same type, quite often don't like each other. Try to get all RAM from the same brand/maker, swap it with someone if you can.