I have been doing this for literally decades...
The fastest CF you can use is 400X (200mb/sec), as that it the limit of the IDE/ATA (166mb/sec) bus is. You can use faster CFs but there will be no improvement in the speed. With certain larger CFs, there are no 400X CFs, only 500X or faster. But that is not an issue. Just keep in mind that the IDE Bus is a bottleneck that CF must slow down for. BTW - a 200X CF is about the same speed as a fast hard drive, so a 266X CF will be a bit faster in booting, writing and loading apps.
Macs can only use the Single CF to IDE Adapter. PCs can use the Dual CF to IDE adapters for 2 CF Cards on the same adapter and you can leave one slot empty for future upgrades. For some using a Dual CF to IDE on a Mac will not work. It just gives you a grey screen after the RAM test - no Happy Mac, no Sad Mac. I do not know why it does this and I gave up trying to figure it out a long time ago. So for Macs - Single CF to IDE Adapters ONLY!
360Alaska posted up correct device - no electronics on it. I have seen Syabus (?sp) Adapters with electronics on them and they work on some Macs and not on others. So to save headaches, these simple no-electronics ones are the best option. Plus they are a lot cheaper. I gotten them from Chinese sellers for $1.50 a piece, but ordering through China means going through customs and that can be a long wait sometimes. For me the longest delivery wait with Chinese sellers was 3 weeks and the shortest was 6 days.
For an Old Mac (OS 7 - OS 9) anything you can put is from 512MB to 32GB are great! for a G3/G4 where you want OSX - you will need 64GB as a good starting point though you can run it on 16GB. But you will need to go into the system prefs and shut off logging of various OSX events. OS7 to OS9 does not write logs like Unix does, so this does not matter. But you need to get Disk Cache to a minimum and turn off virtual memory in the memory control panel.
Good Luck!