My computer had 3 memory cards @ 64MB each for a total of 192 MB.

I got another old computer that had 1 card @ 128 MB, and removed one of the 64MB cards, replacing it with the 128 MB card. That gives me 256 MB of RAM, and that is what shows when I look at computer properties.

My question is: When it boots up, I get the message, "Warning: One or more memory DIMMS are out of rev.
What does this mean, and how is it corrected?

Some computers are picky as to how memory is installed. The usual way is highest MB in bank 1 etc.

With 3 slots, it's doubtful that they need to be installed in pairs.

Try putting the 128 MM in the first bank.

Some computers are picky as to how memory is installed. The usual way is highest MB in bank 1 etc.

With 3 slots, it's doubtful that they need to be installed in pairs.

Try putting the 128 MM in the first bank.

I tried that. No difference.

Does it matter in the operation?

There are a bunch of answers in this search:

Google (http://www.google.com/search?client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&channel=s&hl=en&q=%22out+of+rev.%22+memory+errors&btnG=Google+Search)

I only looked at the first one and got confused.

I read the first one on the Google search, and while I don't understand a lot of the specific details, I get the gist. (I've had similar issues in the past.) I also read some posts on the other forums and seemed like every time they mentioned a Dell. Is that what you have?

You can read over the first hit from keepitsimple's post. The link to Dell's error messages (post #6) is broken. But I just when there and did a search on that error:
Dell - Search - All Dell.com (http://search.dell.com/results.aspx?c=us&l=en&s=gen&cat=sup&k=One+or+more+memory+DIMMS+are+out+of+rev&rpp=12&p=1&rf=all&nk=f&ira=False&~srd=False&ipsys=False&advsrch=False&~ck=tab)

My computer is a Dell.

With a Pentium II, wonder if I would get any benefit from a 1 gig memory card? (If it's available.)

This message is normally associated with RAM speed.
Now, as the error only iccured after you installed the 128 stick, I would say that this is the issue.
Normally this would indicate RAM that was to fast for your system.

To be perfectly honest, I wouldn't bother spending any money on upgrades, as they are likely to be expensive for little gain, due to the age of the machine.
Considering the price of computer hardware, you can pick up a reasonably spec'd machine for little money.

Time to bite to bullet and upgrade completely..

With a Pentium II, wonder if I would get any benefit from a 1 gig memory card? (If it's available.)

There is often a chipset limit for the maximum supported memory or the maximum module size supported. Again given the age, I doubt the system supports a large memory capacity or large module sizes. Dell's website has full product documentation for their system models in the support section which list system specifications and supported hardware options.