How many MB should be on a computer?

Bill Gates once said, "...256 KB...", but that was years ago!

If you are using XP for best performance no less than 1 GB (2 if possible) of RAM (the more the merrier) and vista 2-3 GB.

Also, take your OS into consideration. If you use a 32-bit, then the maximum is 3 - 3.5GB but I wouldn't recommend under 2. For a 64-bit OS, pretty much anything will work. Again, stay over 2gb. The most I've seen in a laptop is the x58 laptop from smooth creations. I think it's 12 GB! The iMac Pro goes up to 32 GB. It'll cost you $6,000 extra just for that. Seriously, unless you're a hard core computer enthusiast/hard core gamer, 4GB will handle virtually anything. Anyhow, those are the bare minimums, but remember, more RAM is better (not counting the cost)!-u will only need 1-2GB of RAM 2 play almost any game and to make ur computer run good for XP but for vista u will need 2-3GB of RAM for a efficient computer.

The 64 bit Windows supports up to 128 GB of RAM but I doubt you can find any hardware that supports that amount. 64 bit actually allows a LOT more, but 128 GB is how much Windows supports. However, a modern PC will not need more than 6 GB, even 6 GB is too much, 4 GB is enough. Your PC will never use 12 GB of RAM unless you do really heavy stuff like HD video editing and a lot of rendering (although is also based a lot on your CPU).

So for a Windows XP, anything above the 1 GB will do.

For Windows vista and 7, atleast 2 GB is recommended.

You do, probably, not ever need more than 6 GB and anything above that is basically a waste of money.

Additional Information:

This is a challenging question to answer - Why? because technology keeps changing year after year. (computers are now using GB, as opposed to MB for memory!) With newer Operating Systems being developed (Windows 7 and Mac OS X) along with faster processors, memory requirements also need to be considered.

You need to ask yourself what are you using your computer for? Are you using if for day to day email and Surfing or are you a power user - graphics, gaming, etc? Take a look at what different companies are suggesting for their laptop/computer bundles? and you can add more memory depending on your usage.

Remember upgrading memory on older computers is dependent on how much the system board is designed to handle. How many slots do you have for memory? If this is what you are considering, you can check a site like crucial.com to check to see how much your computer can be upgraded to.