How do I figure out RAM chips. WORD x BIT to Kilobyte?

[psxphill]

So, I'm trying to put some RAM into my SupraDrive 500XP.
 
The manual says it can take 256k×4 or 1M×4 DRAM chips.

The two numbers are like the dimensions of the grid inside the chip.
To convert ram size to bytes you multiply the two numbers together, this gives you the number of bits.
To convert that number to bytes you then divide by 8.
 
1Mx4 is 512 kilobytes (0.5 of a megabyte) so for 1 megabyte you need 2.
256kx4 is 128k kilobytes (0.125 of a megabyte) so for 1 megabyte you need 8.

[psxphill]

As an aside: does anyone know where I could buy (or trade for) 8MB of 1M x 4 RAM chips for my SupraDrive 500XP? I'm hoping those chips shouldn't me more than 50 cents or so each nowadays.

No idea, I think you'll be disappointed on the pricing though. Someone was selling 256k x 4 for 3 euros a chip.
 
http://www.amibay.com/showthread.php?t=20082
 
Sometimes it's easier to just strip them out of old hardware.

[psxphill]

Not really 3EUR per chip: you forgot to mention that the shipping cost and paypal fees are included

Why would you want to buy them from someone who doesn't then send them to you?

[psxphill]

Only for those he knows (French).
Those who do not:
- try to cheat about the delivery, even if tracking number shows that the delivery is OK.
- ask to be refund by the postoffice, even if tracking number shows that the delivery is OK.

No I mean why would you buy them from someone if you're not going to pay to have them delivered. It's part of the cost.